Error attempting to download subtitles

[ickam]

On attempting to download subtitles I get

./subliminal.py download -l=en -s -f /mnt/films/filmname.mkv
2020-03-12 14:13:25,432 - 16669 - INFO - Found 1 matched file(s).
2020-03-12 14:13:25,434 - 16669 - INFO - Using advanced search mode
2020-03-12 14:13:25,483 - 16669 - ERROR - An error occured processing the language list

I strongly suspect this is related to a problem with subliminal itself that I signalled here: Diaoul/subliminal#994
but I have no idea how to fix it.