The traceback shows that it's in the "if missing_keys" branch of fetch.py's _fetch_everything_for_search. i.e. fetch has decided that that key is either a missing compression parent or missing parent inventory, but the source does not have that key.
The traceback shows that it's in the "if missing_keys" branch of fetch.py's _fetch_ everything_ for_search. i.e. fetch has decided that that key is either a missing compression parent or missing parent inventory, but the source does not have that key.