How do I use null_label and/or null_value in ChoiceFilter for empty values?

[CelestialGuru]

The documentation mentions that null_label (and null_value?) can be used to allow filtering by none stuff as well as real stuff.

https://django-filter.readthedocs.io/en/stable/guide/tips.html#solution-2-using-choicefilter-s-null-choice
https://django-filter.readthedocs.io/en/stable/ref/filters.html#choicefilter

#519 seems to be where this was added but I don't know exactly what it does nor how to use this feature. What I want to know is for ?{a}={b}, what is meant to go for a and what is meant to go in b?

With

class ProductFilter(django_filters.FilterSet):
    category = django_filters.ModelChoiceFilter(
        field_name='category',
        lookup_expr='isnull',
        null_label='Uncategorized',
        queryset=Category.objects.all(),
    )

this is what happens when I try to use it:

• /api/products/?category=1
  • I get ValueError: The QuerySet value for an isnull lookup must be True or False.
• /api/products/?category=Uncategorized
  • I get {"category":["Select a valid choice. That choice is not one of the available choices."]}

What am I missing here?

[CelestialGuru]

I should mention that in my case, category is a ForeignKey, not a ChareField with choices or anything like that.