diff --git a/ICSharpCode.NRefactory.CSharp/Refactoring/TypeSystemAstBuilder.cs b/ICSharpCode.NRefactory.CSharp/Refactoring/TypeSystemAstBuilder.cs
index b514ee82b3..bf5bd3c8c8 100644
--- a/ICSharpCode.NRefactory.CSharp/Refactoring/TypeSystemAstBuilder.cs
+++ b/ICSharpCode.NRefactory.CSharp/Refactoring/TypeSystemAstBuilder.cs
@@ -187,18 +187,19 @@ namespace ICSharpCode.NRefactory.CSharp.Refactoring
 				AddTypeArguments(shortResult, typeArguments, outerTypeParameterCount, typeDef.TypeParameterCount);
 				return shortResult;
 			}
-			
 			MemberType result = new MemberType();
 			if (typeDef.DeclaringTypeDefinition != null) {
 				// check if the declaring type is a nested type in the nested type hierarchy
-				var declaringType = resolver.CurrentTypeDefinition;
-				while (declaringType != null) {
-					if (typeDef.DeclaringTypeDefinition.Equals (declaringType)) {
-						var innerType = new SimpleType(typeDef.Name);
-						AddTypeArguments(innerType, typeArguments, outerTypeParameterCount, typeDef.TypeParameterCount);
-						return innerType;
+				if (resolver != null) {
+					var declaringType = resolver.CurrentTypeDefinition;
+					while (declaringType != null) {
+						if (typeDef.DeclaringTypeDefinition.Equals (declaringType)) {
+							var innerType = new SimpleType(typeDef.Name);
+							AddTypeArguments(innerType, typeArguments, outerTypeParameterCount, typeDef.TypeParameterCount);
+							return innerType;
+						}
+						declaringType = declaringType.DeclaringTypeDefinition;
 					}
-					declaringType = declaringType.DeclaringTypeDefinition;
 				}
 				
 				// Handle nested types
diff --git a/ICSharpCode.NRefactory.Tests/CSharp/Refactoring/TypeSystemAstBuilderTests.cs b/ICSharpCode.NRefactory.Tests/CSharp/Refactoring/TypeSystemAstBuilderTests.cs
index 1d6ceba863..2b32f7e4b5 100644
--- a/ICSharpCode.NRefactory.Tests/CSharp/Refactoring/TypeSystemAstBuilderTests.cs
+++ b/ICSharpCode.NRefactory.Tests/CSharp/Refactoring/TypeSystemAstBuilderTests.cs
@@ -160,8 +160,8 @@ namespace OtherNS {
 		{
 			var type = new ParameterizedType(nestedClass, new[] { compilation.FindType(KnownTypeCode.Char), compilation.FindType(KnownTypeCode.String) });
 			Assert.AreEqual("Base<char>.Nested<string>", TypeToString(type));
-			Assert.AreEqual("Base<char>.Nested<string>", TypeToString(type, baseClass));
-			Assert.AreEqual("Base<char>.Nested<string>", TypeToString(type, nestedClass));
+			Assert.AreEqual("Nested<string>", TypeToString(type, baseClass));
+			Assert.AreEqual("Nested<string>", TypeToString(type, nestedClass));
 			Assert.AreEqual("Base<char>.Nested<string>", TypeToString(type, derivedClass));
 		}