diff --git a/python/mox.py b/python/mox.py
index 257468e..43db021 100755
--- a/python/mox.py
+++ b/python/mox.py
@@ -778,7 +778,7 @@
       rhs: any python object
     """
 
-    raise NotImplementedError, 'method must be implemented by a subclass.'
+    raise NotImplementedError('method must be implemented by a subclass.')
 
   def __eq__(self, rhs):
     return self.equals(rhs)