diff --git a/mercurial/scmutil.py b/mercurial/scmutil.py
index 6d42c797ca6e36ad4d6e20edc9162144c06f44c8_bWVyY3VyaWFsL3NjbXV0aWwucHk=..9da5a2864f3fe169c32abd0150d1c8840b4b556e_bWVyY3VyaWFsL3NjbXV0aWwucHk= 100644
--- a/mercurial/scmutil.py
+++ b/mercurial/scmutil.py
@@ -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))