mercurial/revset.py

--- a/mercurial/revset.py	Wed Sep 17 11:00:09 2014 -0700
+++ b/mercurial/revset.py	Wed Sep 17 10:59:52 2014 -0700
@@ -1198,7 +1198,9 @@
     """
     if x is None:
         p = repo[x].p1().rev()
-        return subset.filter(lambda r: r == p)
+        if p >= 0:
+            return subset & baseset([p])
+        return baseset([])
 
     ps = set()
     cl = repo.changelog