diff -r 6d42c797ca6e -r 9da5a2864f3f mercurial/scmutil.py
--- a/mercurial/scmutil.py	Fri Jun 01 17:37:56 2012 +0200
+++ b/mercurial/scmutil.py	Thu May 31 20:55:30 2012 -0700
@@ -568,7 +568,7 @@
                 newrevs = set(xrange(start, end + step, step))
                 if seen:
                     newrevs.difference_update(seen)
-                    seen.union(newrevs)
+                    seen.update(newrevs)
                 else:
                     seen = newrevs
                 l.extend(sorted(newrevs, reverse=start > end))