--- a/mercurial/revset.py	Tue Jul 10 23:01:53 2018 +0900
+++ b/mercurial/revset.py	Thu Jul 12 23:07:29 2018 +0900
@@ -632,7 +632,10 @@
 
     """
     # only wants the heads of the set passed in
-    for r in heads(repo, fullreposet(repo), x, anyorder):
+    h = heads(repo, fullreposet(repo), x, anyorder)
+    if not h:
+        return baseset()
+    for r in h:
         subset &= dagop.revancestors(repo, baseset([r]))
 
     return subset