Index: Lib/lib-tk/Tkinter.py
===================================================================
--- Lib/lib-tk/Tkinter.py	(revision 59567)
+++ Lib/lib-tk/Tkinter.py	(working copy)
@@ -1918,7 +1918,7 @@
         self.widgetName = widgetName
         BaseWidget._setup(self, master, cnf)
         classes = []
-        for k in cnf.keys():
+        for k in list(cnf.keys()):
             if isinstance(k, type):
                 classes.append((k, cnf[k]))
                 del cnf[k]