--- a/mercurial/revset.py	Mon Jun 13 22:41:45 2016 +0100
+++ b/mercurial/revset.py	Wed Jun 15 20:37:24 2016 +0900
@@ -1890,6 +1890,8 @@
                 'topo.firstbranch can only be used when using the topo sort '
                 'key'))
 
+    if not keys:
+        return revs
     if keys == ["rev"]:
         revs.sort()
         return revs