diff -r f93ff3ab8d14 -r 3e9049876ace mercurial/revset.py
--- a/mercurial/revset.py	Thu Jun 11 14:26:44 2015 -0700
+++ b/mercurial/revset.py	Thu Jun 11 14:27:52 2015 -0700
@@ -614,9 +614,9 @@
     return subset.filter(matches)
 
 def _children(repo, narrow, parentset):
+    if not parentset:
+        return baseset()
     cs = set()
-    if not parentset:
-        return baseset(cs)
     pr = repo.changelog.parentrevs
     # XXX this should be 'parentset.min()' assuming 'parentset' is a smartset
     # (and if it is not, it should.)