diff -r d8c76c82d012 -r 217bfb10e6db mercurial/obsolete.py
--- a/mercurial/obsolete.py	Mon Jul 02 00:31:22 2012 +0200
+++ b/mercurial/obsolete.py	Wed Jul 04 16:37:00 2012 +0200
@@ -182,7 +182,7 @@
             raise ValueError(prec)
         for succ in succs:
             if len(succ) != 20:
-                raise ValueError(prec)
+                raise ValueError(succ)
         marker = (str(prec), tuple(succs), int(flag), encodemeta(metadata))
         self.add(marker)