--- a/mercurial/commands.py	Thu Mar 20 01:35:07 2014 +0100
+++ b/mercurial/commands.py	Mon Apr 07 23:17:51 2014 +0200
@@ -438,8 +438,7 @@
     node = scmutil.revsingle(repo, rev).node()
 
     op1, op2 = repo.dirstate.parents()
-    a = repo.changelog.ancestor(op1, node)
-    if a != node:
+    if node not in repo.changelog.commonancestors(op1, node):
         raise util.Abort(_('cannot backout change that is not an ancestor'))
 
     p1, p2 = repo.changelog.parents(node)