Lite feature complete.
diff --git a/src/ProtocolBuffers.Test/ReflectionTester.cs b/src/ProtocolBuffers.Test/ReflectionTester.cs
index 32b6446..3e7ed39 100644
--- a/src/ProtocolBuffers.Test/ReflectionTester.cs
+++ b/src/ProtocolBuffers.Test/ReflectionTester.cs
@@ -213,7 +213,7 @@
         ExtensionInfo extension = extensionRegistry[field.ContainingType, field.FieldNumber];
         Assert.IsNotNull(extension);
         Assert.IsNotNull(extension.DefaultInstance);
-        return extension.DefaultInstance.WeakCreateBuilderForType();
+        return (IBuilder)extension.DefaultInstance.WeakCreateBuilderForType();
       }
     }