diff --git a/src/DotTiled.Examples/DotTiled.Example.Console/Program.cs b/src/DotTiled.Examples/DotTiled.Example.Console/Program.cs
index 9dba3df..baf36ea 100644
--- a/src/DotTiled.Examples/DotTiled.Example.Console/Program.cs
+++ b/src/DotTiled.Examples/DotTiled.Example.Console/Program.cs
@@ -73,12 +73,12 @@ public class Program
     return templateReader.ReadTemplate();
   }
 
-  private static ICustomTypeDefinition ResolveCustomType(string name)
+  private static Optional<ICustomTypeDefinition> ResolveCustomType(string name)
   {
     ICustomTypeDefinition[] allDefinedTypes =
     [
       new CustomClassDefinition() { Name = "a" },
     ];
-    return allDefinedTypes.FirstOrDefault(type => type.Name == name) ?? throw new InvalidOperationException();
+    return allDefinedTypes.FirstOrDefault(ctd => ctd.Name == name) is ICustomTypeDefinition ctd ? new Optional<ICustomTypeDefinition>(ctd) : Optional<ICustomTypeDefinition>.Empty;
   }
 }
diff --git a/src/DotTiled.Examples/DotTiled.Example.Godot/MapParser.cs b/src/DotTiled.Examples/DotTiled.Example.Godot/MapParser.cs
index 5ad960b..8319d00 100644
--- a/src/DotTiled.Examples/DotTiled.Example.Godot/MapParser.cs
+++ b/src/DotTiled.Examples/DotTiled.Example.Godot/MapParser.cs
@@ -1,4 +1,3 @@
-using System;
 using System.Globalization;
 using System.Linq;
 using DotTiled.Serialization;
@@ -57,12 +56,12 @@ public partial class MapParser : Node2D
     return templateReader.ReadTemplate();
   }
 
-  private static ICustomTypeDefinition ResolveCustomType(string name)
+  private static Optional<ICustomTypeDefinition> ResolveCustomType(string name)
   {
     ICustomTypeDefinition[] allDefinedTypes =
       [
         new CustomClassDefinition() { Name = "a" },
       ];
-    return allDefinedTypes.FirstOrDefault(type => type.Name == name) ?? throw new InvalidOperationException();
+    return allDefinedTypes.FirstOrDefault(ctd => ctd.Name == name) is ICustomTypeDefinition ctd ? new Optional<ICustomTypeDefinition>(ctd) : Optional<ICustomTypeDefinition>.Empty;
   }
 }
diff --git a/src/DotTiled.Tests/UnitTests/Serialization/LoaderTests.cs b/src/DotTiled.Tests/UnitTests/Serialization/LoaderTests.cs
index d54ab9f..7edff12 100644
--- a/src/DotTiled.Tests/UnitTests/Serialization/LoaderTests.cs
+++ b/src/DotTiled.Tests/UnitTests/Serialization/LoaderTests.cs
@@ -246,7 +246,7 @@ public class LoaderTests
   }
 
   [Fact]
-  public void LoadMap_MapHasClassAndLoaderHasNoCustomTypes_ThrowsException()
+  public void LoadMap_MapHasClassAndLoaderHasNoCustomTypes_ReturnsMapWithEmptyProperties()
   {
     // Arrange
     var resourceReader = Substitute.For<IResourceReader>();
@@ -270,8 +270,11 @@ public class LoaderTests
     var customTypeDefinitions = Enumerable.Empty<ICustomTypeDefinition>();
     var loader = new Loader(resourceReader, resourceCache, customTypeDefinitions);
 
-    // Act & Assert
-    Assert.Throws<KeyNotFoundException>(() => loader.LoadMap("map.tmx"));
+    // Act
+    var result = loader.LoadMap("map.tmx");
+
+    // Assert
+    DotTiledAssert.AssertProperties([], result.Properties);
   }
 
   [Fact]
diff --git a/src/DotTiled.Tests/UnitTests/Serialization/MapReaderTests.cs b/src/DotTiled.Tests/UnitTests/Serialization/MapReaderTests.cs
index 885f57e..dd6bcca 100644
--- a/src/DotTiled.Tests/UnitTests/Serialization/MapReaderTests.cs
+++ b/src/DotTiled.Tests/UnitTests/Serialization/MapReaderTests.cs
@@ -32,9 +32,14 @@ public partial class MapReaderTests
         using var tilesetReader = new TilesetReader(tilesetString, ResolveTileset, ResolveTemplate, ResolveCustomType);
         return tilesetReader.ReadTileset();
       }
-      ICustomTypeDefinition ResolveCustomType(string name)
+      Optional<ICustomTypeDefinition> ResolveCustomType(string name)
       {
-        return customTypeDefinitions.FirstOrDefault(ctd => ctd.Name == name)!;
+        if (customTypeDefinitions.FirstOrDefault(ctd => ctd.Name == name) is ICustomTypeDefinition ctd)
+        {
+          return new Optional<ICustomTypeDefinition>(ctd);
+        }
+
+        return Optional<ICustomTypeDefinition>.Empty;
       }
       using var mapReader = new MapReader(mapString, ResolveTileset, ResolveTemplate, ResolveCustomType);
 
diff --git a/src/DotTiled.Tests/UnitTests/Serialization/Tmj/TmjMapReaderTests.cs b/src/DotTiled.Tests/UnitTests/Serialization/Tmj/TmjMapReaderTests.cs
index a896a48..aa289f6 100644
--- a/src/DotTiled.Tests/UnitTests/Serialization/Tmj/TmjMapReaderTests.cs
+++ b/src/DotTiled.Tests/UnitTests/Serialization/Tmj/TmjMapReaderTests.cs
@@ -28,9 +28,14 @@ public partial class TmjMapReaderTests
       using var tilesetReader = new TsjTilesetReader(tilesetJson, ResolveTileset, ResolveTemplate, ResolveCustomType);
       return tilesetReader.ReadTileset();
     }
-    ICustomTypeDefinition ResolveCustomType(string name)
+    Optional<ICustomTypeDefinition> ResolveCustomType(string name)
     {
-      return customTypeDefinitions.FirstOrDefault(ctd => ctd.Name == name)!;
+      if (customTypeDefinitions.FirstOrDefault(ctd => ctd.Name == name) is ICustomTypeDefinition ctd)
+      {
+        return new Optional<ICustomTypeDefinition>(ctd);
+      }
+
+      return Optional<ICustomTypeDefinition>.Empty;
     }
     using var mapReader = new TmjMapReader(json, ResolveTileset, ResolveTemplate, ResolveCustomType);
 
diff --git a/src/DotTiled.Tests/UnitTests/Serialization/Tmx/TmxMapReaderTests.cs b/src/DotTiled.Tests/UnitTests/Serialization/Tmx/TmxMapReaderTests.cs
index b6e5813..82dba07 100644
--- a/src/DotTiled.Tests/UnitTests/Serialization/Tmx/TmxMapReaderTests.cs
+++ b/src/DotTiled.Tests/UnitTests/Serialization/Tmx/TmxMapReaderTests.cs
@@ -28,9 +28,14 @@ public partial class TmxMapReaderTests
       using var tilesetReader = new TsxTilesetReader(xmlTilesetReader, ResolveTileset, ResolveTemplate, ResolveCustomType);
       return tilesetReader.ReadTileset();
     }
-    ICustomTypeDefinition ResolveCustomType(string name)
+    Optional<ICustomTypeDefinition> ResolveCustomType(string name)
     {
-      return customTypeDefinitions.FirstOrDefault(ctd => ctd.Name == name)!;
+      if (customTypeDefinitions.FirstOrDefault(ctd => ctd.Name == name) is ICustomTypeDefinition ctd)
+      {
+        return new Optional<ICustomTypeDefinition>(ctd);
+      }
+
+      return Optional<ICustomTypeDefinition>.Empty;
     }
     using var mapReader = new TmxMapReader(reader, ResolveTileset, ResolveTemplate, ResolveCustomType);
 
diff --git a/src/DotTiled/Serialization/Helpers.cs b/src/DotTiled/Serialization/Helpers.cs
index e784b80..d716293 100644
--- a/src/DotTiled/Serialization/Helpers.cs
+++ b/src/DotTiled/Serialization/Helpers.cs
@@ -86,13 +86,16 @@ internal static partial class Helpers
     };
   }
 
-  internal static List<IProperty> ResolveClassProperties(string className, Func<string, ICustomTypeDefinition> customTypeResolver)
+  internal static List<IProperty> ResolveClassProperties(string className, Func<string, Optional<ICustomTypeDefinition>> customTypeResolver)
   {
     if (string.IsNullOrWhiteSpace(className))
       return null;
 
     var customType = customTypeResolver(className) ?? throw new InvalidOperationException($"Could not resolve custom type '{className}'.");
-    if (customType is not CustomClassDefinition ccd)
+    if (!customType.HasValue)
+      return null;
+
+    if (customType.Value is not CustomClassDefinition ccd)
       throw new InvalidOperationException($"Custom type '{className}' is not a class.");
 
     return CreateInstanceOfCustomClass(ccd, customTypeResolver);
@@ -100,17 +103,31 @@ internal static partial class Helpers
 
   internal static List<IProperty> CreateInstanceOfCustomClass(
     CustomClassDefinition customClassDefinition,
-    Func<string, ICustomTypeDefinition> customTypeResolver)
+    Func<string, Optional<ICustomTypeDefinition>> customTypeResolver)
   {
     return customClassDefinition.Members.Select(x =>
     {
       if (x is ClassProperty cp)
       {
+        var resolvedType = customTypeResolver(cp.PropertyType);
+        if (!resolvedType.HasValue)
+        {
+          return new ClassProperty
+          {
+            Name = cp.Name,
+            PropertyType = cp.PropertyType,
+            Value = []
+          };
+        }
+
+        if (resolvedType.Value is not CustomClassDefinition ccd)
+          throw new InvalidOperationException($"Custom type '{cp.PropertyType}' is not a class.");
+
         return new ClassProperty
         {
           Name = cp.Name,
           PropertyType = cp.PropertyType,
-          Value = CreateInstanceOfCustomClass((CustomClassDefinition)customTypeResolver(cp.PropertyType), customTypeResolver)
+          Value = CreateInstanceOfCustomClass(ccd, customTypeResolver)
         };
       }
 
diff --git a/src/DotTiled/Serialization/Loader.cs b/src/DotTiled/Serialization/Loader.cs
index 02e258c..dd4fc2a 100644
--- a/src/DotTiled/Serialization/Loader.cs
+++ b/src/DotTiled/Serialization/Loader.cs
@@ -12,7 +12,7 @@ public class Loader
 {
   private readonly IResourceReader _resourceReader;
   private readonly IResourceCache _resourceCache;
-  private readonly IDictionary<string, ICustomTypeDefinition> _customTypeDefinitions;
+  private readonly Dictionary<string, ICustomTypeDefinition> _customTypeDefinitions;
 
   /// <summary>
   /// Initializes a new instance of the <see cref="Loader"/> class with the given <paramref name="resourceReader"/>, <paramref name="resourceCache"/>, and <paramref name="customTypeDefinitions"/>.
@@ -114,5 +114,11 @@ public class Loader
         return templateReader.ReadTemplate();
       });
 
-  private ICustomTypeDefinition CustomTypeResolver(string name) => _customTypeDefinitions[name];
+  private Optional<ICustomTypeDefinition> CustomTypeResolver(string name)
+  {
+    if (_customTypeDefinitions.TryGetValue(name, out var customTypeDefinition))
+      return new Optional<ICustomTypeDefinition>(customTypeDefinition);
+
+    return Optional<ICustomTypeDefinition>.Empty;
+  }
 }
diff --git a/src/DotTiled/Serialization/MapReader.cs b/src/DotTiled/Serialization/MapReader.cs
index d202e8f..5a6f6a7 100644
--- a/src/DotTiled/Serialization/MapReader.cs
+++ b/src/DotTiled/Serialization/MapReader.cs
@@ -14,7 +14,7 @@ public class MapReader : IMapReader
   // External resolvers
   private readonly Func<string, Tileset> _externalTilesetResolver;
   private readonly Func<string, Template> _externalTemplateResolver;
-  private readonly Func<string, ICustomTypeDefinition> _customTypeResolver;
+  private readonly Func<string, Optional<ICustomTypeDefinition>> _customTypeResolver;
 
   private readonly StringReader _mapStringReader;
   private readonly XmlReader _xmlReader;
@@ -33,7 +33,7 @@ public class MapReader : IMapReader
     string map,
     Func<string, Tileset> externalTilesetResolver,
     Func<string, Template> externalTemplateResolver,
-    Func<string, ICustomTypeDefinition> customTypeResolver)
+    Func<string, Optional<ICustomTypeDefinition>> customTypeResolver)
   {
     _externalTilesetResolver = externalTilesetResolver ?? throw new ArgumentNullException(nameof(externalTilesetResolver));
     _externalTemplateResolver = externalTemplateResolver ?? throw new ArgumentNullException(nameof(externalTemplateResolver));
diff --git a/src/DotTiled/Serialization/TemplateReader.cs b/src/DotTiled/Serialization/TemplateReader.cs
index bf210c0..d44da3e 100644
--- a/src/DotTiled/Serialization/TemplateReader.cs
+++ b/src/DotTiled/Serialization/TemplateReader.cs
@@ -14,7 +14,7 @@ public class TemplateReader : ITemplateReader
   // External resolvers
   private readonly Func<string, Tileset> _externalTilesetResolver;
   private readonly Func<string, Template> _externalTemplateResolver;
-  private readonly Func<string, ICustomTypeDefinition> _customTypeResolver;
+  private readonly Func<string, Optional<ICustomTypeDefinition>> _customTypeResolver;
 
   private readonly StringReader _templateStringReader;
   private readonly XmlReader _xmlReader;
@@ -33,7 +33,7 @@ public class TemplateReader : ITemplateReader
     string template,
     Func<string, Tileset> externalTilesetResolver,
     Func<string, Template> externalTemplateResolver,
-    Func<string, ICustomTypeDefinition> customTypeResolver)
+    Func<string, Optional<ICustomTypeDefinition>> customTypeResolver)
   {
     _externalTilesetResolver = externalTilesetResolver ?? throw new ArgumentNullException(nameof(externalTilesetResolver));
     _externalTemplateResolver = externalTemplateResolver ?? throw new ArgumentNullException(nameof(externalTemplateResolver));
diff --git a/src/DotTiled/Serialization/TilesetReader.cs b/src/DotTiled/Serialization/TilesetReader.cs
index 180d269..3d7c83a 100644
--- a/src/DotTiled/Serialization/TilesetReader.cs
+++ b/src/DotTiled/Serialization/TilesetReader.cs
@@ -14,7 +14,7 @@ public class TilesetReader : ITilesetReader
   // External resolvers
   private readonly Func<string, Tileset> _externalTilesetResolver;
   private readonly Func<string, Template> _externalTemplateResolver;
-  private readonly Func<string, ICustomTypeDefinition> _customTypeResolver;
+  private readonly Func<string, Optional<ICustomTypeDefinition>> _customTypeResolver;
 
   private readonly StringReader _tilesetStringReader;
   private readonly XmlReader _xmlReader;
@@ -33,7 +33,7 @@ public class TilesetReader : ITilesetReader
     string tileset,
     Func<string, Tileset> externalTilesetResolver,
     Func<string, Template> externalTemplateResolver,
-    Func<string, ICustomTypeDefinition> customTypeResolver)
+    Func<string, Optional<ICustomTypeDefinition>> customTypeResolver)
   {
     _externalTilesetResolver = externalTilesetResolver ?? throw new ArgumentNullException(nameof(externalTilesetResolver));
     _externalTemplateResolver = externalTemplateResolver ?? throw new ArgumentNullException(nameof(externalTemplateResolver));
diff --git a/src/DotTiled/Serialization/Tmj/TjTemplateReader.cs b/src/DotTiled/Serialization/Tmj/TjTemplateReader.cs
index 792bb73..ef80970 100644
--- a/src/DotTiled/Serialization/Tmj/TjTemplateReader.cs
+++ b/src/DotTiled/Serialization/Tmj/TjTemplateReader.cs
@@ -19,7 +19,7 @@ public class TjTemplateReader : TmjReaderBase, ITemplateReader
     string jsonString,
     Func<string, Tileset> externalTilesetResolver,
     Func<string, Template> externalTemplateResolver,
-    Func<string, ICustomTypeDefinition> customTypeResolver) : base(
+    Func<string, Optional<ICustomTypeDefinition>> customTypeResolver) : base(
       jsonString, externalTilesetResolver, externalTemplateResolver, customTypeResolver)
   { }
 
diff --git a/src/DotTiled/Serialization/Tmj/TmjMapReader.cs b/src/DotTiled/Serialization/Tmj/TmjMapReader.cs
index 8ceb211..cffa036 100644
--- a/src/DotTiled/Serialization/Tmj/TmjMapReader.cs
+++ b/src/DotTiled/Serialization/Tmj/TmjMapReader.cs
@@ -19,7 +19,7 @@ public class TmjMapReader : TmjReaderBase, IMapReader
     string jsonString,
     Func<string, Tileset> externalTilesetResolver,
     Func<string, Template> externalTemplateResolver,
-    Func<string, ICustomTypeDefinition> customTypeResolver) : base(
+    Func<string, Optional<ICustomTypeDefinition>> customTypeResolver) : base(
       jsonString, externalTilesetResolver, externalTemplateResolver, customTypeResolver)
   { }
 
diff --git a/src/DotTiled/Serialization/Tmj/TmjReaderBase.Properties.cs b/src/DotTiled/Serialization/Tmj/TmjReaderBase.Properties.cs
index b877382..1ffff1c 100644
--- a/src/DotTiled/Serialization/Tmj/TmjReaderBase.Properties.cs
+++ b/src/DotTiled/Serialization/Tmj/TmjReaderBase.Properties.cs
@@ -1,3 +1,4 @@
+using System;
 using System.Collections.Generic;
 using System.Globalization;
 using System.Linq;
@@ -63,21 +64,38 @@ public abstract partial class TmjReaderBase
     var propertyType = element.GetRequiredProperty<string>("propertytype");
     var customTypeDef = _customTypeResolver(propertyType);
 
-    if (customTypeDef is CustomClassDefinition ccd)
+    // If the custom class definition is not found,
+    // we assume an empty class definition.
+    if (!customTypeDef.HasValue)
     {
-      var propsInType = Helpers.CreateInstanceOfCustomClass(ccd, _customTypeResolver);
-      var props = element.GetOptionalPropertyCustom<List<IProperty>>("value", e => ReadPropertiesInsideClass(e, ccd)).GetValueOr([]);
-      var mergedProps = Helpers.MergeProperties(propsInType, props);
+      if (!element.TryGetProperty("value", out var valueElement))
+        return new ClassProperty { Name = name, PropertyType = propertyType, Value = [] };
 
       return new ClassProperty
       {
         Name = name,
         PropertyType = propertyType,
-        Value = mergedProps
+        Value = ReadPropertiesInsideClass(valueElement, new CustomClassDefinition
+        {
+          Name = propertyType,
+          Members = []
+        })
       };
     }
 
-    throw new JsonException($"Unknown custom class '{propertyType}'.");
+    if (customTypeDef.Value is not CustomClassDefinition ccd)
+      throw new JsonException($"Custom type {propertyType} is not a class.");
+
+    var propsInType = Helpers.CreateInstanceOfCustomClass(ccd, _customTypeResolver);
+    var props = element.GetOptionalPropertyCustom<List<IProperty>>("value", e => ReadPropertiesInsideClass(e, ccd)).GetValueOr([]);
+    var mergedProps = Helpers.MergeProperties(propsInType, props);
+
+    return new ClassProperty
+    {
+      Name = name,
+      PropertyType = propertyType,
+      Value = mergedProps
+    };
   }
 
   internal List<IProperty> ReadPropertiesInsideClass(
@@ -91,6 +109,33 @@ public abstract partial class TmjReaderBase
       if (!element.TryGetProperty(prop.Name, out var propElement))
         continue;
 
+      if (prop is ClassProperty classProp)
+      {
+        var resolvedCustomType = _customTypeResolver(classProp.PropertyType);
+        if (!resolvedCustomType.HasValue)
+        {
+          resultingProps.Add(new ClassProperty
+          {
+            Name = classProp.Name,
+            PropertyType = classProp.PropertyType,
+            Value = []
+          });
+          continue;
+        }
+
+        if (resolvedCustomType.Value is not CustomClassDefinition ccd)
+          throw new JsonException($"Custom type '{classProp.PropertyType}' is not a class.");
+
+        var readProps = ReadPropertiesInsideClass(propElement, ccd);
+        resultingProps.Add(new ClassProperty
+        {
+          Name = classProp.Name,
+          PropertyType = classProp.PropertyType,
+          Value = readProps
+        });
+        continue;
+      }
+
       IProperty property = prop.Type switch
       {
         PropertyType.String => new StringProperty { Name = prop.Name, Value = propElement.GetValueAs<string>() },
@@ -100,8 +145,8 @@ public abstract partial class TmjReaderBase
         PropertyType.Color => new ColorProperty { Name = prop.Name, Value = Color.Parse(propElement.GetValueAs<string>(), CultureInfo.InvariantCulture) },
         PropertyType.File => new FileProperty { Name = prop.Name, Value = propElement.GetValueAs<string>() },
         PropertyType.Object => new ObjectProperty { Name = prop.Name, Value = propElement.GetValueAs<uint>() },
-        PropertyType.Class => new ClassProperty { Name = prop.Name, PropertyType = ((ClassProperty)prop).PropertyType, Value = ReadPropertiesInsideClass(propElement, (CustomClassDefinition)_customTypeResolver(((ClassProperty)prop).PropertyType)) },
         PropertyType.Enum => ReadEnumProperty(propElement),
+        PropertyType.Class => throw new NotImplementedException("Class properties should be handled elsewhere"),
         _ => throw new JsonException("Invalid property type")
       };
 
@@ -115,7 +160,7 @@ public abstract partial class TmjReaderBase
   {
     var name = element.GetRequiredProperty<string>("name");
     var propertyType = element.GetRequiredProperty<string>("propertytype");
-    var typeInXml = element.GetOptionalPropertyParseable<PropertyType>("type", (s) => s switch
+    var typeInJson = element.GetOptionalPropertyParseable<PropertyType>("type", (s) => s switch
     {
       "string" => PropertyType.String,
       "int" => PropertyType.Int,
@@ -123,8 +168,24 @@ public abstract partial class TmjReaderBase
     }).GetValueOr(PropertyType.String);
     var customTypeDef = _customTypeResolver(propertyType);
 
-    if (customTypeDef is not CustomEnumDefinition ced)
-      throw new JsonException($"Unknown custom enum '{propertyType}'. Enums must be defined");
+    if (!customTypeDef.HasValue)
+    {
+      if (typeInJson == PropertyType.String)
+      {
+        var value = element.GetRequiredProperty<string>("value");
+        var values = value.Split(',').Select(v => v.Trim()).ToHashSet();
+        return new EnumProperty { Name = name, PropertyType = propertyType, Value = values };
+      }
+      else
+      {
+        var value = element.GetRequiredProperty<int>("value");
+        var values = new HashSet<string> { value.ToString(CultureInfo.InvariantCulture) };
+        return new EnumProperty { Name = name, PropertyType = propertyType, Value = values };
+      }
+    }
+
+    if (customTypeDef.Value is not CustomEnumDefinition ced)
+      throw new JsonException($"Custom type '{propertyType}' is not an enum.");
 
     if (ced.StorageType == CustomEnumStorageType.String)
     {
diff --git a/src/DotTiled/Serialization/Tmj/TmjReaderBase.cs b/src/DotTiled/Serialization/Tmj/TmjReaderBase.cs
index f32100a..c63b913 100644
--- a/src/DotTiled/Serialization/Tmj/TmjReaderBase.cs
+++ b/src/DotTiled/Serialization/Tmj/TmjReaderBase.cs
@@ -13,7 +13,7 @@ public abstract partial class TmjReaderBase : IDisposable
   // External resolvers
   private readonly Func<string, Tileset> _externalTilesetResolver;
   private readonly Func<string, Template> _externalTemplateResolver;
-  private readonly Func<string, ICustomTypeDefinition> _customTypeResolver;
+  private readonly Func<string, Optional<ICustomTypeDefinition>> _customTypeResolver;
 
   /// <summary>
   /// The root element of the JSON document being read.
@@ -34,7 +34,7 @@ public abstract partial class TmjReaderBase : IDisposable
     string jsonString,
     Func<string, Tileset> externalTilesetResolver,
     Func<string, Template> externalTemplateResolver,
-    Func<string, ICustomTypeDefinition> customTypeResolver)
+    Func<string, Optional<ICustomTypeDefinition>> customTypeResolver)
   {
     RootElement = JsonDocument.Parse(jsonString ?? throw new ArgumentNullException(nameof(jsonString))).RootElement;
     _externalTilesetResolver = externalTilesetResolver ?? throw new ArgumentNullException(nameof(externalTilesetResolver));
diff --git a/src/DotTiled/Serialization/Tmj/TsjTilesetReader.cs b/src/DotTiled/Serialization/Tmj/TsjTilesetReader.cs
index 5816c8a..87f4b59 100644
--- a/src/DotTiled/Serialization/Tmj/TsjTilesetReader.cs
+++ b/src/DotTiled/Serialization/Tmj/TsjTilesetReader.cs
@@ -19,7 +19,7 @@ public class TsjTilesetReader : TmjReaderBase, ITilesetReader
     string jsonString,
     Func<string, Tileset> externalTilesetResolver,
     Func<string, Template> externalTemplateResolver,
-    Func<string, ICustomTypeDefinition> customTypeResolver) : base(
+    Func<string, Optional<ICustomTypeDefinition>> customTypeResolver) : base(
       jsonString, externalTilesetResolver, externalTemplateResolver, customTypeResolver)
   { }
 
diff --git a/src/DotTiled/Serialization/Tmx/TmxMapReader.cs b/src/DotTiled/Serialization/Tmx/TmxMapReader.cs
index 6029481..e993a4c 100644
--- a/src/DotTiled/Serialization/Tmx/TmxMapReader.cs
+++ b/src/DotTiled/Serialization/Tmx/TmxMapReader.cs
@@ -16,7 +16,7 @@ public class TmxMapReader : TmxReaderBase, IMapReader
     XmlReader reader,
     Func<string, Tileset> externalTilesetResolver,
     Func<string, Template> externalTemplateResolver,
-    Func<string, ICustomTypeDefinition> customTypeResolver) : base(
+    Func<string, Optional<ICustomTypeDefinition>> customTypeResolver) : base(
       reader, externalTilesetResolver, externalTemplateResolver, customTypeResolver)
   { }
 
diff --git a/src/DotTiled/Serialization/Tmx/TmxReaderBase.Properties.cs b/src/DotTiled/Serialization/Tmx/TmxReaderBase.Properties.cs
index bfdb93d..4aec221 100644
--- a/src/DotTiled/Serialization/Tmx/TmxReaderBase.Properties.cs
+++ b/src/DotTiled/Serialization/Tmx/TmxReaderBase.Properties.cs
@@ -1,4 +1,5 @@
 using System.Collections.Generic;
+using System.Globalization;
 using System.Linq;
 using System.Xml;
 
@@ -66,25 +67,35 @@ public abstract partial class TmxReaderBase
     var propertyType = _reader.GetRequiredAttribute("propertytype");
     var customTypeDef = _customTypeResolver(propertyType);
 
-    if (customTypeDef is CustomClassDefinition ccd)
+    // If the custom class definition is not found,
+    // we assume an empty class definition.
+    if (!customTypeDef.HasValue)
     {
       if (!_reader.IsEmptyElement)
       {
         _reader.ReadStartElement("property");
-        var propsInType = Helpers.CreateInstanceOfCustomClass(ccd, _customTypeResolver);
         var props = ReadProperties();
-        var mergedProps = Helpers.MergeProperties(propsInType, props);
         _reader.ReadEndElement();
-        return new ClassProperty { Name = name, PropertyType = propertyType, Value = mergedProps };
-      }
-      else
-      {
-        var propsInType = Helpers.CreateInstanceOfCustomClass(ccd, _customTypeResolver);
-        return new ClassProperty { Name = name, PropertyType = propertyType, Value = propsInType };
+        return new ClassProperty { Name = name, PropertyType = propertyType, Value = props };
       }
+
+      return new ClassProperty { Name = name, PropertyType = propertyType, Value = [] };
     }
 
-    throw new XmlException($"Unkonwn custom class definition: {propertyType}");
+    if (customTypeDef.Value is not CustomClassDefinition ccd)
+      throw new XmlException($"Custom type {propertyType} is not a class.");
+
+    var propsInType = Helpers.CreateInstanceOfCustomClass(ccd, _customTypeResolver);
+    if (!_reader.IsEmptyElement)
+    {
+      _reader.ReadStartElement("property");
+      var props = ReadProperties();
+      var mergedProps = Helpers.MergeProperties(propsInType, props);
+      _reader.ReadEndElement();
+      return new ClassProperty { Name = name, PropertyType = propertyType, Value = mergedProps };
+    }
+
+    return new ClassProperty { Name = name, PropertyType = propertyType, Value = propsInType };
   }
 
   internal EnumProperty ReadEnumProperty()
@@ -99,8 +110,26 @@ public abstract partial class TmxReaderBase
     }) ?? PropertyType.String;
     var customTypeDef = _customTypeResolver(propertyType);
 
-    if (customTypeDef is not CustomEnumDefinition ced)
-      throw new XmlException($"Unknown custom enum definition: {propertyType}. Enums must be defined");
+    // If the custom enum definition is not found,
+    // we assume an empty enum definition.
+    if (!customTypeDef.HasValue)
+    {
+      if (typeInXml == PropertyType.String)
+      {
+        var value = _reader.GetRequiredAttribute("value");
+        var values = value.Split(',').Select(v => v.Trim()).ToHashSet();
+        return new EnumProperty { Name = name, PropertyType = propertyType, Value = values };
+      }
+      else
+      {
+        var value = _reader.GetRequiredAttributeParseable<int>("value");
+        var values = new HashSet<string> { value.ToString(CultureInfo.InvariantCulture) };
+        return new EnumProperty { Name = name, PropertyType = propertyType, Value = values };
+      }
+    }
+
+    if (customTypeDef.Value is not CustomEnumDefinition ced)
+      throw new XmlException($"Custom defined type {propertyType} is not an enum.");
 
     if (ced.StorageType == CustomEnumStorageType.String)
     {
@@ -144,6 +173,6 @@ public abstract partial class TmxReaderBase
       }
     }
 
-    throw new XmlException($"Unknown custom enum storage type: {ced.StorageType}");
+    throw new XmlException($"Unable to read enum property {name} with type {propertyType}");
   }
 }
diff --git a/src/DotTiled/Serialization/Tmx/TmxReaderBase.cs b/src/DotTiled/Serialization/Tmx/TmxReaderBase.cs
index e30af82..e3fe2d0 100644
--- a/src/DotTiled/Serialization/Tmx/TmxReaderBase.cs
+++ b/src/DotTiled/Serialization/Tmx/TmxReaderBase.cs
@@ -11,7 +11,7 @@ public abstract partial class TmxReaderBase : IDisposable
   // External resolvers
   private readonly Func<string, Tileset> _externalTilesetResolver;
   private readonly Func<string, Template> _externalTemplateResolver;
-  private readonly Func<string, ICustomTypeDefinition> _customTypeResolver;
+  private readonly Func<string, Optional<ICustomTypeDefinition>> _customTypeResolver;
 
   private readonly XmlReader _reader;
   private bool disposedValue;
@@ -28,7 +28,7 @@ public abstract partial class TmxReaderBase : IDisposable
     XmlReader reader,
     Func<string, Tileset> externalTilesetResolver,
     Func<string, Template> externalTemplateResolver,
-    Func<string, ICustomTypeDefinition> customTypeResolver)
+    Func<string, Optional<ICustomTypeDefinition>> customTypeResolver)
   {
     _reader = reader ?? throw new ArgumentNullException(nameof(reader));
     _externalTilesetResolver = externalTilesetResolver ?? throw new ArgumentNullException(nameof(externalTilesetResolver));
diff --git a/src/DotTiled/Serialization/Tmx/TsxTilesetReader.cs b/src/DotTiled/Serialization/Tmx/TsxTilesetReader.cs
index f0dbcc9..195f767 100644
--- a/src/DotTiled/Serialization/Tmx/TsxTilesetReader.cs
+++ b/src/DotTiled/Serialization/Tmx/TsxTilesetReader.cs
@@ -16,7 +16,7 @@ public class TsxTilesetReader : TmxReaderBase, ITilesetReader
     XmlReader reader,
     Func<string, Tileset> externalTilesetResolver,
     Func<string, Template> externalTemplateResolver,
-    Func<string, ICustomTypeDefinition> customTypeResolver) : base(
+    Func<string, Optional<ICustomTypeDefinition>> customTypeResolver) : base(
       reader, externalTilesetResolver, externalTemplateResolver, customTypeResolver)
   { }
 
diff --git a/src/DotTiled/Serialization/Tmx/TxTemplateReader.cs b/src/DotTiled/Serialization/Tmx/TxTemplateReader.cs
index 72da9dc..8a9d3b1 100644
--- a/src/DotTiled/Serialization/Tmx/TxTemplateReader.cs
+++ b/src/DotTiled/Serialization/Tmx/TxTemplateReader.cs
@@ -16,7 +16,7 @@ public class TxTemplateReader : TmxReaderBase, ITemplateReader
     XmlReader reader,
     Func<string, Tileset> externalTilesetResolver,
     Func<string, Template> externalTemplateResolver,
-    Func<string, ICustomTypeDefinition> customTypeResolver) : base(
+    Func<string, Optional<ICustomTypeDefinition>> customTypeResolver) : base(
       reader, externalTilesetResolver, externalTemplateResolver, customTypeResolver)
   { }