diff -r 96ee8ca99f5a -r 89e11a6da785 mercurial/changegroup.py
--- a/mercurial/changegroup.py	Mon May 03 12:23:27 2021 +0200
+++ b/mercurial/changegroup.py	Mon May 03 12:23:37 2021 +0200
@@ -804,10 +804,10 @@
                 # We failed to resolve a parent for this node, so
                 # we crash the changegroup construction.
                 if util.safehasattr(store, 'target'):
-                    target = store._indexfile
+                    target = store.display_id
                 else:
                     # some revlog not actually a revlog
-                    target = store._revlog._indexfile
+                    target = store._revlog.display_id
 
                 raise error.Abort(
                     b"unable to resolve parent while packing '%s' %r"