NullBooleanField default to unknown

I have a NullBooleanField that I would like to show up in my admin
interface as radio buttons with unknown selected by default.

# I have the following simplified files

## models.py ##
class Value(models.Model):
presence = models.NullBooleanField(default=False)

## admin.py ##
PRESENCE_CHOICES = ((True, 'True'), (False, 'False'), (None, 'None') )
class ValueAdmin(admin.ModelAdmin):
formfield_overrides = {
models.NullBooleanField: {'widget':
RadioSelect(choices=PRESENCE_CHOICES)}
}
admin.site.register(Value, ValueAdmin)

This behaves very closely to what I desire, but I don't know what I
could replace the False with in:
presence = models.NullBooleanField(default=False)
in order to make it behave as I would like.

Thanks so much for your time,
Ben R.

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