Kill more lingering string exceptions in Lib/plat-mac/.
diff --git a/Lib/plat-mac/MiniAEFrame.py b/Lib/plat-mac/MiniAEFrame.py
index 8e551a5..eed7428 100644
--- a/Lib/plat-mac/MiniAEFrame.py
+++ b/Lib/plat-mac/MiniAEFrame.py
@@ -141,7 +141,8 @@
         elif ('****', '****') in self.ae_handlers:
             _function = self.ae_handlers[('****', '****')]
         else:
-            raise 'Cannot happen: AE callback without handler', (_class, _type)
+            raise RuntimeError('AE callback without handler: '
+                               + str((_class, _type)))
 
         # XXXX Do key-to-name mapping here