diff -r 0961a7eb82c4 -r 7eaad1101242 hgext/evolve.py
--- a/hgext/evolve.py	Thu Jan 09 21:46:23 2014 -0800
+++ b/hgext/evolve.py	Mon Jan 20 10:57:10 2014 +0100
@@ -767,7 +767,10 @@
     try:
         rebase = extensions.find('rebase')
         # dummy state to trick rebase node
-        assert orig.p2().rev() == node.nullrev, 'no support yet'
+        if not orig.p2().rev() == node.nullrev:
+            raise util.Abort(
+                'no support for evolution merge changesets yet',
+                hint="Redo the merge a use `hg prune` to obsolete the old one")
         destbookmarks = repo.nodebookmarks(dest.node())
         cmdutil.duplicatecopies(repo, orig.node(), dest.node())
         nodesrc = orig.node()