--- a/mercurial/cmdutil.py	Tue Sep 28 13:59:01 2021 -0700
+++ b/mercurial/cmdutil.py	Tue Sep 28 15:11:22 2021 -0700
@@ -1480,7 +1480,7 @@
             # TODO: Remove this restriction and make it also create the copy
             #       targets (and remove the rename source if rename==True).
             raise error.InputError(_(b'--at-rev requires --after'))
-        ctx = scmutil.revsingle(repo, rev)
+        ctx = logcmdutil.revsingle(repo, rev)
         if len(ctx.parents()) > 1:
             raise error.InputError(
                 _(b'cannot mark/unmark copy in merge commit')