beet duplicates tiebreak by flexible attribute

[JOJ0]

I'm trying to get beet duplicates tiebreak on behalf of a flexible attribute that is not present on all items. I'd like to keep the files that do have this attribute and let beet duplicate decide to delete the other version.

I'm getting this:

$ beet duplicates -k artist -k title amadou
Traceback (most recent call last):
  File "/Users/jojo/git/beets/beets/library.py", line 701, in __getitem__
    return super().__getitem__(key)
  File "/Users/jojo/git/beets/beets/dbcore/db.py", line 452, in __getitem__
    return self._get(key, raise_=True)
  File "/Users/jojo/git/beets/beets/dbcore/db.py", line 442, in _get
    raise KeyError(key)
KeyError: 'mood_acoustic'

having this configured:

duplicates:
    tiebreak:
        items:
            #- bitrate
            #- spin
            - mood_acoustic
    format: '$isrc|$data_source|$mb_albumid  $albumartist--$album--$title :::$path'