diff --git a/DotTiled.Tests/Serialization/TestData.cs b/DotTiled.Tests/Serialization/TestData.cs
index 9b3b13f..8fc9b9e 100644
--- a/DotTiled.Tests/Serialization/TestData.cs
+++ b/DotTiled.Tests/Serialization/TestData.cs
@@ -27,4 +27,44 @@ public static partial class TestData
     using var stringReader = new StreamReader(stream);
     return stringReader.ReadToEnd();
   }
+
+  public static IEnumerable<object[]> MapsThatHaveTmxAndTmj =>
+  [
+    ["Serialization.TestData.Map.default_map.default-map", TestData.DefaultMap(), Array.Empty<CustomTypeDefinition>()]
+  ];
+
+  private static CustomTypeDefinition[] typedefs = [
+    new CustomClassDefinition
+    {
+      Name = "TestClass",
+      ID = 1,
+      UseAs = CustomClassUseAs.Property,
+      Members = [
+        new StringProperty
+        {
+          Name = "Name",
+          Value = ""
+        },
+        new FloatProperty
+        {
+          Name = "Amount",
+          Value = 0f
+        }
+      ]
+    },
+    new CustomClassDefinition
+    {
+      Name = "Test",
+      ID = 2,
+      UseAs = CustomClassUseAs.All,
+      Members = [
+        new ClassProperty
+        {
+          Name = "Yep",
+          PropertyType = "TestClass",
+          Properties = []
+        }
+      ]
+    }
+  ];
 }
diff --git a/DotTiled.Tests/Serialization/Tmj/TmjMapReaderTests.cs b/DotTiled.Tests/Serialization/Tmj/TmjMapReaderTests.cs
index e04423b..0cdad90 100644
--- a/DotTiled.Tests/Serialization/Tmj/TmjMapReaderTests.cs
+++ b/DotTiled.Tests/Serialization/Tmj/TmjMapReaderTests.cs
@@ -2,51 +2,16 @@ namespace DotTiled.Tests;
 
 public partial class TmjMapReaderTests
 {
-  public static IEnumerable<object[]> DeserializeMap_ValidTmjExternalTilesetsAndTemplates_ReturnsMapThatEqualsExpected_Data =>
-  [
-    ["Serialization.TestData.Map.default_map.default-map.tmj", TestData.DefaultMap()]
-  ];
-
+  public static IEnumerable<object[]> Maps => TestData.MapsThatHaveTmxAndTmj;
   [Theory]
-  [MemberData(nameof(DeserializeMap_ValidTmjExternalTilesetsAndTemplates_ReturnsMapThatEqualsExpected_Data))]
-  public void TmxMapReaderReadMap_ValidTmjExternalTilesetsAndTemplates_ReturnsMapThatEqualsExpected(string testDataFile, Map expectedMap)
+  [MemberData(nameof(Maps))]
+  public void TmxMapReaderReadMap_ValidTmjExternalTilesetsAndTemplates_ReturnsMapThatEqualsExpected(
+    string testDataFile,
+    Map expectedMap,
+    IReadOnlyCollection<CustomTypeDefinition> customTypeDefinitions)
   {
     // Arrange
-    CustomTypeDefinition[] customTypeDefinitions = [
-      new CustomClassDefinition
-      {
-        Name = "TestClass",
-        ID = 1,
-        UseAs = CustomClassUseAs.Property,
-        Members = [
-          new StringProperty
-          {
-            Name = "Name",
-            Value = ""
-          },
-          new FloatProperty
-          {
-            Name = "Amount",
-            Value = 0f
-          }
-        ]
-      },
-      new CustomClassDefinition
-      {
-        Name = "Test",
-        ID = 2,
-        UseAs = CustomClassUseAs.All,
-        Members = [
-          new ClassProperty
-          {
-            Name = "Yep",
-            PropertyType = "TestClass",
-            Properties = []
-          }
-        ]
-      }
-    ];
-
+    testDataFile += ".tmj";
     var json = TestData.GetRawStringFor(testDataFile);
     Template ResolveTemplate(string source)
     {
diff --git a/DotTiled.Tests/Serialization/Tmx/TmxMapReaderTests.cs b/DotTiled.Tests/Serialization/Tmx/TmxMapReaderTests.cs
index 5ee162f..3134863 100644
--- a/DotTiled.Tests/Serialization/Tmx/TmxMapReaderTests.cs
+++ b/DotTiled.Tests/Serialization/Tmx/TmxMapReaderTests.cs
@@ -4,50 +4,16 @@ namespace DotTiled.Tests;
 
 public partial class TmxMapReaderTests
 {
-  public static IEnumerable<object[]> DeserializeMap_ValidXmlExternalTilesetsAndTemplates_ReturnsMapThatEqualsExpected_Data =>
-  [
-    ["Serialization.TestData.Map.default_map.default-map.tmx", TestData.DefaultMap()]
-  ];
-
+  public static IEnumerable<object[]> Maps => TestData.MapsThatHaveTmxAndTmj;
   [Theory]
-  [MemberData(nameof(DeserializeMap_ValidXmlExternalTilesetsAndTemplates_ReturnsMapThatEqualsExpected_Data))]
-  public void TmxMapReaderReadMap_ValidXmlExternalTilesetsAndTemplates_ReturnsMapThatEqualsExpected(string testDataFile, Map expectedMap)
+  [MemberData(nameof(Maps))]
+  public void TmxMapReaderReadMap_ValidXmlExternalTilesetsAndTemplates_ReturnsMapThatEqualsExpected(
+    string testDataFile,
+    Map expectedMap,
+    IReadOnlyCollection<CustomTypeDefinition> customTypeDefinitions)
   {
     // Arrange
-    CustomTypeDefinition[] customTypeDefinitions = [
-      new CustomClassDefinition
-      {
-        Name = "TestClass",
-        ID = 1,
-        UseAs = CustomClassUseAs.Property,
-        Members = [
-          new StringProperty
-          {
-            Name = "Name",
-            Value = ""
-          },
-          new FloatProperty
-          {
-            Name = "Amount",
-            Value = 0f
-          }
-        ]
-      },
-      new CustomClassDefinition
-      {
-        Name = "Test",
-        ID = 2,
-        UseAs = CustomClassUseAs.All,
-        Members = [
-          new ClassProperty
-          {
-            Name = "Yep",
-            PropertyType = "TestClass",
-            Properties = []
-          }
-        ]
-      }
-    ];
+    testDataFile += ".tmx";
     using var reader = TestData.GetXmlReaderFor(testDataFile);
     Template ResolveTemplate(string source)
     {