Index: Lib/lib-tk/tkMessageBox.py
===================================================================
--- Lib/lib-tk/tkMessageBox.py	(revision 73494)
+++ Lib/lib-tk/tkMessageBox.py	(working copy)
@@ -70,11 +70,13 @@
     if title:   options["title"] = title
     if message: options["message"] = message
     res = Message(**options).show()
-    # In some Tcl installations, Tcl converts yes/no into a boolean
+    # In some Tcl installations, yes/no is converted into a boolean.
     if isinstance(res, bool):
-        if res: return YES
+        if res:
+            return YES
         return NO
-    return res
+    # In others we get a Tcl_Obj.
+    return str(res)
 
 def showinfo(title=None, message=None, **options):
     "Show an info message"