--- a/mercurial/context.py	Sat Jul 04 12:40:34 2009 +0200
+++ b/mercurial/context.py	Fri Jul 03 10:43:46 2009 -0400
@@ -199,7 +199,9 @@
 
         assert (changeid is not None
                 or fileid is not None
-                or changectx is not None)
+                or changectx is not None), \
+                ("bad args: changeid=%r, fileid=%r, changectx=%r"
+                 % (changeid, fileid, changectx))
 
         if filelog:
             self._filelog = filelog