diff -r b237c1c6dc19 -r 31493a1b0e39 hgext3rd/topic/revset.py
--- a/hgext3rd/topic/revset.py	Thu Oct 26 09:03:48 2017 +0200
+++ b/hgext3rd/topic/revset.py	Sun Oct 15 00:07:21 2017 +0530
@@ -72,11 +72,10 @@
     run)."""
     err = 'stack() takes no argument, it works on current topic'
     revset.getargs(x, 0, 0, err)
-    topic = repo.currenttopic
     topic = None
     branch = None
-    if not topic and repo.currenttopic:
+    if repo.currenttopic:
         topic = repo.currenttopic
-    if not topic:
+    else:
         branch = repo[None].branch()
     return revset.baseset(stack.stack(repo, branch=branch, topic=topic)[1:]) & subset