mercurial/subrepo.py

--- a/mercurial/subrepo.py	Mon Apr 19 20:38:52 2021 -0400
+++ b/mercurial/subrepo.py	Mon Mar 29 01:52:06 2021 +0200
@@ -21,7 +21,6 @@
 from .node import (
     bin,
     hex,
-    nullid,
     short,
 )
 from . import (
@@ -686,7 +685,7 @@
         # we can't fully delete the repository as it may contain
         # local-only history
         self.ui.note(_(b'removing subrepo %s\n') % subrelpath(self))
-        hg.clean(self._repo, nullid, False)
+        hg.clean(self._repo, self._repo.nullid, False)
 
     def _get(self, state):
         source, revision, kind = state