diff -r 77c52ce50e6a -r 911e25dc9d8c mercurial/revset.py
--- a/mercurial/revset.py	Thu Jul 18 17:07:34 2019 +0800
+++ b/mercurial/revset.py	Thu Jul 18 17:10:38 2019 +0800
@@ -1695,7 +1695,7 @@
     parent. (EXPERIMENTAL)
     """
     if x is None:
-        stacks = stackmod.getstack(repo, x)
+        stacks = stackmod.getstack(repo)
     else:
         stacks = smartset.baseset([])
         for revision in getset(repo, fullreposet(repo), x):