Flac import fails if no MusicBrainz-ID present

I'm getting this error on some flac-files when I try to import them: Error while importing $file.flac: TypeError 'NoneType' object is not subscriptable

The affected files aren't present in the MusicBrainz database and so have none of their Ids. They do have Tite, Artist and Album tagged correctly. This seems to only affect flac-import.