pytags crashes on --add, --set

OS ubuntu 12.04 64 bit
frits@fritsubuntu:~/tests$ pytags --set genre=Baroc Allegro.m4a
Allegro.m4a
/usr/lib/python2.7/dist-packages/pytagsfs/metastore/mutagen_.py:77: UnicodeWarning: Unicode equal comparison failed to convert both arguments to Unicode - interpreting them as being unequal
  if key in super(TranslatedMP4, self).keys():
Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/sclapp/main.py", line 201, in main
    status = realmain(argv)
  File "/usr/lib/python2.7/dist-packages/pytagsfs/pytags.py", line 192, in main
    return update_tags(values.format, values.operations, args)
  File "/usr/lib/python2.7/dist-packages/pytagsfs/pytags.py", line 104, in update_tags
    tags.save()
  File "/usr/lib/python2.7/dist-packages/mutagen/__init__.py", line 132, in save
    return self.tags.save(filename, **kwargs)
  File "/usr/lib/python2.7/dist-packages/mutagen/mp4.py", line 277, in save
    values.append(info[1](self, key, value, *info[2:]))
MP4MetadataValueError: 'NoneType' object is not callable

Something bad happened, and is most likely a bug.
Please file a bug report to the author.
Include the error message(s) printed here.
exiting with status 255

mp3 and flac crash as well.