diff --git a/Lib/idlelib/macosxSupport.py b/Lib/idlelib/macosxSupport.py
index e759157..b23221e 100644
--- a/Lib/idlelib/macosxSupport.py
+++ b/Lib/idlelib/macosxSupport.py
@@ -82,6 +82,12 @@
 
     def config_dialog(event=None):
         from idlelib import configDialog
+
+        # Ensure that the root object has an instance_dict attribute,
+        # mirrors code in EditorWindow (although that sets the attribute
+        # on an EditorWindow instance that is then passed as the first
+        # argument to ConfigDialog)
+        root.instance_dict = flist.inversedict
         configDialog.ConfigDialog(root, 'Settings')