Convert ends with ffmpeg error

[uwoehler]

I'm trying to convert a bunch of mp3 files to a m4b. Conversion starts, but at the end ffmpeg throws an error:
Stream map '0:v' matches no streams.
How can I solve this?

[yermak]

quick fix: add any image as artwork.
Proper fix will be released tomorrow.

[yermak]

Duplicates #359