Docker could not create bridge line

A relay operator reported on Tor Forum that running docker exec CONTAINER_ID get-bridge-line is returning this error:

user@dockerhost:~$ docker exec torbridge_obfs4-bridge_1 get-bridge-line

Could not create bridge line. Tor's log format may have changed. This is a bug.

https://forum.torproject.net/t/help-censored-users-run-a-tor-bridge/704/17