diff --git a/src/Libraries/ICSharpCode.Build.Tasks/Project/MonoToolLocationHelper.cs b/src/Libraries/ICSharpCode.Build.Tasks/Project/MonoToolLocationHelper.cs
index 038976821b..0167d57598 100644
--- a/src/Libraries/ICSharpCode.Build.Tasks/Project/MonoToolLocationHelper.cs
+++ b/src/Libraries/ICSharpCode.Build.Tasks/Project/MonoToolLocationHelper.cs
@@ -172,7 +172,9 @@ namespace ICSharpCode.Build.Tasks
 				if (key != null) {
 					string readValue = (string)key.GetValue(name);
 					key.Close();
-					return readValue;
+					if (readValue != null) {
+						return readValue;
+					}
 				}
 			} catch (SecurityException) { }