diff --git a/Lib/idlelib/macosx_main.py b/Lib/idlelib/macosx_main.py
index 88e4197..78480fa 100644
--- a/Lib/idlelib/macosx_main.py
+++ b/Lib/idlelib/macosx_main.py
@@ -39,7 +39,7 @@
 
     # this module will become the namepsace used by the interactive
     # interpreter; remove all variables we have defined.
-    del sys, __file__, boolcheck, split, join
+    del sys, __file__, boolcheck, split, join, isdir
     __import__('run').main()
 else:
     # Load idlelib/idle.py which starts the application.