diff -r f48cd2f48d14 -r 85ec2a55fe7c hgext/evolve.py
--- a/hgext/evolve.py	Mon Jun 30 13:37:15 2014 -0400
+++ b/hgext/evolve.py	Mon Jun 30 13:38:49 2014 -0400
@@ -2135,13 +2135,15 @@
 
     roots = repo.revs('roots(%ld)', revs)
     if len(roots) > 1:
-        raise util.Abort("set has multiple roots")
+        raise util.Abort(_("cannot fold non-linear revisions "
+                           "(multiple roots given)"))
     root = repo[roots[0]]
     if root.phase() <= phases.public:
         raise util.Abort(_("cannot fold public revisions"))
     heads = repo.revs('heads(%ld)', revs)
     if len(heads) > 1:
-        raise util.Abort("set has multiple heads")
+        raise util.Abort(_("cannot fold non-linear revisions "
+                           "(multiple heads given)"))
     head = repo[heads[0]]
     wlock = lock = None
     try: