--- bitpim-1.0.7+dfsg1.orig/src/bpcalendar.py
+++ bitpim-1.0.7+dfsg1/src/bpcalendar.py
@@ -1328,7 +1328,7 @@
         elif from_version==3 and to_version==2:
             return self._convert3to2(dict, ringtone_index)
         else:
-            raise 'Invalid conversion'
+            raise NotImplementedError('Invalid conversion')
 
     def _convert2to3(self, dict, ringtone_index):
         """