diff --git a/ICSharpCode.NRefactory/TypeSystem/ExtensionMethods.cs b/ICSharpCode.NRefactory/TypeSystem/ExtensionMethods.cs
index e746c401c4..b39e75b8b7 100644
--- a/ICSharpCode.NRefactory/TypeSystem/ExtensionMethods.cs
+++ b/ICSharpCode.NRefactory/TypeSystem/ExtensionMethods.cs
@@ -258,7 +258,7 @@ namespace ICSharpCode.NRefactory.TypeSystem
 		/// <summary>
 		/// Gets all sub type definitions defined in a context.
 		/// </summary>
-		public static IEnumerable<ITypeDefinition> GetSubTypeDefinitions (this IType baseType, ITypeResolveContext context)
+		public static IEnumerable<ITypeDefinition> GetSubTypeDefinitions (this ITypeDefinition baseType, ITypeResolveContext context)
 		{
 			foreach (var contextType in context.GetAllClasses ()) {
 				if (contextType.IsDerivedFrom (baseType, context))