diff --git a/hgext/rebase.py b/hgext/rebase.py
--- a/hgext/rebase.py
+++ b/hgext/rebase.py
@@ -696,6 +696,7 @@
             editor = cmdutil.getcommiteditor(edit=editopt, editform=editform)
             revtoreuse = max(self.state)
 
+            self.wctx.setparents(repo[p1].node(), repo[self.external].node())
             newnode = self._concludenode(
                 revtoreuse, p1, self.external, editor, commitmsg=commitmsg
             )
@@ -1433,7 +1434,6 @@
     if b'branch' in extra:
         branch = extra[b'branch']
 
-    wctx.setparents(repo[p1].node(), repo[p2].node())
     memctx = wctx.tomemctx(
         commitmsg,
         date=date,
@@ -1454,8 +1454,6 @@
     if not repo.ui.configbool(b'rebase', b'singletransaction'):
         dsguard = dirstateguard.dirstateguard(repo, b'rebase')
     with dsguard:
-        repo.setparents(repo[p1].node(), repo[p2].node())
-
         # Commit might fail if unresolved files exist
         newnode = repo.commit(
             text=commitmsg, user=user, date=date, extra=extra, editor=editor
diff --git a/tests/test-copytrace-heuristics.t b/tests/test-copytrace-heuristics.t
--- a/tests/test-copytrace-heuristics.t
+++ b/tests/test-copytrace-heuristics.t
@@ -290,6 +290,7 @@
 
   $ hg rebase -s 1 -d 2
   rebasing 1:472e38d57782 "mv a b"
+  warning: can't find ancestor for 'b' copied from 'a'!
   saved backup bundle to $TESTTMP/repo/.hg/strip-backup/472e38d57782-17d50e29-rebase.hg
   $ hg up -q c492ed3c7e35dcd1dc938053b8adf56e2cfbd062
   $ ls