diff --git a/src/PolyType/ReflectionProvider/ReflectionTypeShapeProvider.cs b/src/PolyType/ReflectionProvider/ReflectionTypeShapeProvider.cs
index 64a0febb..2c10cfc7 100644
--- a/src/PolyType/ReflectionProvider/ReflectionTypeShapeProvider.cs
+++ b/src/PolyType/ReflectionProvider/ReflectionTypeShapeProvider.cs
@@ -294,13 +294,13 @@ private IUnionTypeShape CreateUnionTypeShape(Type unionType, FSharpUnionInfo? fS
             return (IUnionTypeShape)Activator.CreateInstance(fsharpUnionTypeTy, fSharpUnionInfo, this, options)!;
         }
 
-        List<DerivedTypeShapeAttribute> derivedTypeAttributes = unionType.GetCustomAttributes<DerivedTypeShapeAttribute>().ToList();
+        List<DerivedTypeShapeAttribute> derivedTypeAttributes = unionType.GetCustomAttributes<DerivedTypeShapeAttribute>(inherit: false).ToList();
 
         // Honor KnownTypeAttribute annotations only when no DerivedTypeShapeAttribute is present,
         // which takes precedence over KnownTypeAttribute.
         if (derivedTypeAttributes.Count == 0)
         {
-            var mappedKnownTypeAttributes = unionType.GetCustomAttributes<KnownTypeAttribute>()
+            var mappedKnownTypeAttributes = unionType.GetCustomAttributes<KnownTypeAttribute>(inherit: false)
                 .Select(attr =>
                 {
                     if (attr.Type is null)
@@ -308,8 +308,9 @@ private IUnionTypeShape CreateUnionTypeShape(Type unionType, FSharpUnionInfo? fS
                         throw new NotSupportedException("KnownTypeAttribute annotations using methods are not supported.");
                     }
 
-                    return new DerivedTypeShapeAttribute(attr.Type);
-                });
+                    return attr.Type;
+                })
+                .Select(t => new DerivedTypeShapeAttribute(t));
 
             derivedTypeAttributes.AddRange(mappedKnownTypeAttributes);
         }
diff --git a/tests/PolyType.Tests/DataContractShapeTests.cs b/tests/PolyType.Tests/DataContractShapeTests.cs
index 79929bed..4ff8d850 100644
--- a/tests/PolyType.Tests/DataContractShapeTests.cs
+++ b/tests/PolyType.Tests/DataContractShapeTests.cs
@@ -89,9 +89,19 @@ public void KnownTypeAttribute_ReportsUnionShape()
     {
         var shape = Assert.IsType<IUnionTypeShape>(providerUnderTest.Provider.GetTypeShape(typeof(Animal)), exactMatch: false);
 
-        Assert.Equal(2, shape.UnionCases.Count);
+        Assert.Equal(3, shape.UnionCases.Count);
         Assert.Contains(shape.UnionCases, c => c.UnionCaseType.Type == typeof(Dog));
         Assert.Contains(shape.UnionCases, c => c.UnionCaseType.Type == typeof(Cat));
+        Assert.Contains(shape.UnionCases, c => c.UnionCaseType.Type == typeof(PersianCat));
+    }
+
+    [Fact]
+    public void KnownTypeAttribute_DerivedTypesReportsUnionShape()
+    {
+        var shape = Assert.IsType<IUnionTypeShape>(providerUnderTest.Provider.GetTypeShape(typeof(Cat)), exactMatch: false);
+
+        Assert.Single(shape.UnionCases);
+        Assert.Contains(shape.UnionCases, c => c.UnionCaseType.Type == typeof(PersianCat));
     }
 
     [Fact]
@@ -193,6 +203,7 @@ public enum ContractEnum
     [DataContract]
     [KnownType(typeof(Dog))]
     [KnownType(typeof(Cat))]
+    [KnownType(typeof(PersianCat))]
     public partial class Animal
     {
         [DataMember(Order = 0)] public string? Name { get; set; }
@@ -204,12 +215,21 @@ public class Dog : Animal
         [DataMember(Order = 1)] public bool Barks { get; set; }
     }
 
+    [GenerateShape]
     [DataContract]
-    public class Cat : Animal
+    [KnownType(typeof(PersianCat))]
+    public partial class Cat : Animal
     {
         [DataMember(Order = 1)] public int Lives { get; set; }
     }
 
+    [DataContract]
+    public class PersianCat : Cat
+    {
+        [DataMember(Order = 2)] public string? FurColor { get; set; }
+        [DataMember(Order = 3)] public bool RequiresGrooming { get; set; }
+    }
+
     [GenerateShape]
     [KnownType(typeof(DogNoDataContract))]
     public partial class AnimalNoDataContract