diff --git a/Source/Additions/NSMutableString+GNUstepBase.m b/Source/Additions/NSMutableString+GNUstepBase.m
index b519d6cf6..d3efd6ade 100644
--- a/Source/Additions/NSMutableString+GNUstepBase.m
+++ b/Source/Additions/NSMutableString+GNUstepBase.m
@@ -168,6 +168,11 @@
   return [_parent isEqualToString: anObject];
 }
 
+- (BOOL) isProxy
+{
+  return YES;
+}
+
 - (NSUInteger) length
 {
   return [_parent length];