diff -r b6ef1395d77f -r 8abee656e14c mercurial/scmutil.py
--- a/mercurial/scmutil.py	Fri Jun 01 15:13:05 2012 -0500
+++ b/mercurial/scmutil.py	Fri Jun 01 15:14:29 2012 -0500
@@ -569,7 +569,7 @@
                 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))