diff -r b84d27386285 -r 20a5dd5d6dd9 mercurial/graphmod.py
--- a/mercurial/graphmod.py	Fri Sep 05 17:28:37 2008 +0200
+++ b/mercurial/graphmod.py	Fri Sep 12 16:15:01 2008 +0200
@@ -27,6 +27,7 @@
     """
 
     assert start_rev >= stop_rev
+    assert stop_rev >= 0
     curr_rev = start_rev
     revs = []
     cl = repo.changelog