diff --git a/DecoratorGenerator.UnitTests/Tests.cs b/DecoratorGenerator.UnitTests/Tests.cs
index f3218bb..6d42358 100644
--- a/DecoratorGenerator.UnitTests/Tests.cs
+++ b/DecoratorGenerator.UnitTests/Tests.cs
@@ -5,6 +5,7 @@
 using SampleLibrary.Deep.Nesteds;
 using System.Reflection;
 using System.Text;
+using TestLibrary;
 using VerifyCS = DecoratorGenerator.UnitTests.CSharpSourceGeneratorVerifier<DecoratorGenerator.Main>;
 
 namespace DecoratorGenerator.UnitTests;
@@ -41,6 +42,29 @@ public async Task OneInterface() {
         }.RunAsync();
     }
 
+    [Test]
+    public async Task OneInterface_Internal() {
+        var source = await ReadCSharpFile<IInternalType>(true);
+        var generated = await ReadCSharpFile<InternalTypeDecorator>(true);
+
+        await new VerifyCS.Test
+        {
+            TestState = {
+                ReferenceAssemblies = ReferenceAssemblies.Net.Net90,
+                AdditionalReferences =
+                {
+                    implementationAssembly,
+                    GetAssembly("TestLibrary")
+                },
+                Sources = { source },
+                GeneratedSources =
+                {
+                    (typeof(Main), "InternalTypeDecorator.generated.cs", SourceText.From(generated, Encoding.UTF8, SourceHashAlgorithm.Sha256)),
+                },
+            },
+        }.RunAsync();
+    }
+
     [Test]
     public async Task OneInterface_Properties() {
         var source = await ReadCSharpFile<ILionProperties>(true);
diff --git a/DecoratorGenerator/OutputGenerator.cs b/DecoratorGenerator/OutputGenerator.cs
index 79fc462..d69989a 100644
--- a/DecoratorGenerator/OutputGenerator.cs
+++ b/DecoratorGenerator/OutputGenerator.cs
@@ -8,6 +8,7 @@ namespace DecoratorGenerator
     internal static class OutputGenerator
     {
         public static (string source, string className) GenerateOutputs(INamedTypeSymbol @interface) {
+            var interfaceAccessModifier = @interface.DeclaredAccessibility.ToString().ToLower();
             var className = $"{@interface.Name.Substring(1)}Decorator";
             var formattedConstraintTypes = FormatInterfaceConstraintTypes(@interface);
             var formattedConstraints = CreateFormattedConstraints(@interface.TypeParameters);
@@ -25,7 +26,7 @@ public static (string source, string className) GenerateOutputs(INamedTypeSymbol
 #nullable restore
 namespace {@interface.ContainingNamespace.ToDisplayString()};
 
-public abstract class {className}{formattedConstraintTypes} : {@interface.Name}{formattedConstraintTypes}{(formattedConstraints != string.Empty ? $@" {formattedConstraints}" : string.Empty)}
+{interfaceAccessModifier} abstract class {className}{formattedConstraintTypes} : {@interface.Name}{formattedConstraintTypes}{(formattedConstraints != string.Empty ? $@" {formattedConstraints}" : string.Empty)}
 {{
     private {@interface.Name}{formattedConstraintTypes} {targetFieldName};
 
diff --git a/TestLibrary/IInternalType.cs b/TestLibrary/IInternalType.cs
new file mode 100644
index 0000000..c833b9e
--- /dev/null
+++ b/TestLibrary/IInternalType.cs
@@ -0,0 +1,9 @@
+using DecoratorGenerator;
+
+namespace TestLibrary;
+
+[Decorate]
+internal interface IInternalType
+{
+
+}
diff --git a/TestLibrary/InternalTypeDecorator.generated.cs b/TestLibrary/InternalTypeDecorator.generated.cs
new file mode 100644
index 0000000..11e3400
--- /dev/null
+++ b/TestLibrary/InternalTypeDecorator.generated.cs
@@ -0,0 +1,16 @@
+// <auto-generated/>
+#nullable restore
+namespace TestLibrary;
+
+internal abstract class InternalTypeDecorator : IInternalType
+{
+    private IInternalType internalType;
+
+    protected InternalTypeDecorator(IInternalType internalType) {
+        this.internalType = internalType;
+    }
+
+
+
+
+}