[Python] Django haystack with xapian backend: filtering for empty MultiValueField

I have an Index with this field:

tags=indexes.MultiValueField(null=True)

def prepare_tags(self, obj):
return obj.data.get('tags', [])


How do I use SearchQuerySet to filter for empty tags?

Nething of what I've tried works:

events=SearchQuerySet().models(Event)
events.filter(tags=[])
events.filter(tags='')
events.filter(tags=None) # Silly
events.filter(tags__in=[]) # Weird


While there are entries with empty tags:

(pdb) ev1=[_ for _ in events if not(_.object.data.get('tags', []))]
(pdb) len(ev1)
922


Or even on the haystack level:

(pdb) len([_ for _ in events if not(_.tags)])
922

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