diff -r c04e5e937139 -r 9f12d8665c7b mercurial/merge.py
--- a/mercurial/merge.py	Fri Apr 25 13:44:51 2014 -0700
+++ b/mercurial/merge.py	Wed Apr 30 14:19:01 2014 -0500
@@ -992,7 +992,7 @@
                 cahs = repo.changelog.commonancestorsheads(p1.node(), p2.node())
                 pas = [repo[anc] for anc in (sorted(cahs) or [nullid])]
             else:
-                pas = [p1.ancestor(p2)]
+                pas = [p1.ancestor(p2, warn=True)]
 
         fp1, fp2, xp1, xp2 = p1.node(), p2.node(), str(p1), str(p2)