--- a/hgext3rd/topic/revset.py	Thu Jun 29 00:11:07 2017 +0530
+++ b/hgext3rd/topic/revset.py	Thu Jun 29 01:58:26 2017 +0530
@@ -71,6 +71,8 @@
     This is roughly equivalent to 'topic(.) - obsolete' with a sorting moving
     unstable changeset after there future parent (as if evolve where already
     run)."""
+    err = 'stack() takes no argument, it works on current topic'
+    revset.getargs(x, 0, 0, err)
     topic = repo.currenttopic
     if not topic:
         raise error.Abort(_('no active topic to list'))