AssertionError in pre-receive hook

Occasionally a git import on launchpad fails with something along the lines of:
2020-03-24 14:58:24 INFO Everything up-to-date
2020-03-24 15:05:56 INFO remote: Traceback (most recent call last):
2020-03-24 15:05:56 INFO remote: File "hooks/pre-receive", line 148, in <module>
2020-03-24 15:05:56 INFO remote: rule_lines = check_ref_permissions(sock, rpc_key, ref_paths)
2020-03-24 15:05:56 INFO remote: File "hooks/pre-receive", line 130, in check_ref_permissions
2020-03-24 15:05:56 INFO remote: {'key': rpc_key, 'paths': ref_paths})
2020-03-24 15:05:56 INFO remote: File "hooks/pre-receive", line 120, in rpc_invoke
2020-03-24 15:05:56 INFO remote: res = json.loads(netstring_recv(sock))
2020-03-24 15:05:56 INFO remote: File "hooks/pre-receive", line 104, in netstring_recv
2020-03-24 15:05:56 INFO remote: assert c.isdigit()
2020-03-24 15:05:56 INFO remote: AssertionError

(from http://launchpadlibrarian.net/470506969/registry-systemd-+git-systemd.log)

Not sure what the cause is.