diff --git a/src/Main/Base/Test/MemberLookupHelperTests.cs b/src/Main/Base/Test/MemberLookupHelperTests.cs
index 42843e05cf..73c3d61923 100644
--- a/src/Main/Base/Test/MemberLookupHelperTests.cs
+++ b/src/Main/Base/Test/MemberLookupHelperTests.cs
@@ -153,6 +153,24 @@ namespace ICSharpCode.SharpDevelop.Tests
 			Assert.AreEqual("System.Windows.Forms.ToolStripItem", res.FullyQualifiedName);
 		}
 		
+		[Test]
+		public void GetCommonTypeOfStringAndNull()
+		{
+			IReturnType res = MemberLookupHelper.GetCommonType(msc,
+			                                                   msc.GetClass("System.String", 0).DefaultReturnType,
+			                                                   NullReturnType.Instance);
+			Assert.AreEqual("System.String", res.FullyQualifiedName);
+		}
+		
+		[Test]
+		public void GetCommonTypeOfNullAndString()
+		{
+			IReturnType res = MemberLookupHelper.GetCommonType(msc,
+			                                                   NullReturnType.Instance,
+			                                                   msc.GetClass("System.String", 0).DefaultReturnType);
+			Assert.AreEqual("System.String", res.FullyQualifiedName);
+		}
+		
 		[Test]
 		public void GetTypeInheritanceTreeOfClassDerivingFromListOfString()
 		{
diff --git a/src/Main/Base/Test/OverloadFinding.cs b/src/Main/Base/Test/OverloadFinding.cs
index 683dc02fb5..781a15629d 100644
--- a/src/Main/Base/Test/OverloadFinding.cs
+++ b/src/Main/Base/Test/OverloadFinding.cs
@@ -47,6 +47,18 @@ namespace ICSharpCode.SharpDevelop.Tests
 			Test("(long.MaxValue)", 0, overloads);
 		}
 		
+		[Test] public void NullForReferenceTypes()
+		{
+			string[] overloads = {"(int a)", "(string a)"};
+			Test("(null)", 1, overloads);
+		}
+		
+		[Test] public void NullForNullableType()
+		{
+			string[] overloads = {"(int a)", "(int? a)"};
+			Test("(null)", 1, overloads);
+		}
+		
 		NRefactoryResolverTests nrrt = new NRefactoryResolverTests();
 		
 		void Test(string callExpr, int num, params string[] signatures)
diff --git a/src/Main/ICSharpCode.SharpDevelop.Dom/Project/Src/MemberLookupHelper.cs b/src/Main/ICSharpCode.SharpDevelop.Dom/Project/Src/MemberLookupHelper.cs
index 91649491fa..9371cc0172 100644
--- a/src/Main/ICSharpCode.SharpDevelop.Dom/Project/Src/MemberLookupHelper.cs
+++ b/src/Main/ICSharpCode.SharpDevelop.Dom/Project/Src/MemberLookupHelper.cs
@@ -691,8 +691,6 @@ namespace ICSharpCode.SharpDevelop.Dom
 		
 		public static bool IsApplicable(IReturnType argument, IReturnType expected)
 		{
-			if (argument == null || argument == NullReturnType.Instance)
-				return true; // "null" can be passed for any argument
 			return ConversionExistsInternal(argument, expected, true);
 		}
 		#endregion
@@ -748,6 +746,20 @@ namespace ICSharpCode.SharpDevelop.Dom
 			if (toIsDefault && to.FullyQualifiedName == "System.Object") {
 				return true; // from any type to object
 			}
+			if (from == NullReturnType.Instance) {
+				IClass toClass = to.GetUnderlyingClass();
+				if (toClass != null) {
+					switch (toClass.ClassType) {
+						case ClassType.Class:
+						case ClassType.Delegate:
+						case ClassType.Interface:
+							return true;
+						case ClassType.Struct:
+							return toClass.FullyQualifiedName == "System.Nullable";
+					}
+				}
+				return false;
+			}
 			
 			if ((toIsDefault || to.IsConstructedReturnType || to.IsGenericReturnType)
 			    && (fromIsDefault || from.IsArrayReturnType || from.IsConstructedReturnType))