diff --git a/mercurial/revset.py b/mercurial/revset.py
--- a/mercurial/revset.py
+++ b/mercurial/revset.py
@@ -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):