Why is BooleanField non-null?

[jobevers]

I had a model:

class MyModel(EmbeddedDocument):
    myField = BooleanField(null=True)

and was getting an error about "Cannot return null for non-nullable". It seemed that it didn't matter if I set null=True, null=False. Digging into this, I found:

graphene-mongo/graphene_mongo/converter.py

Lines 39 to 41 in fbdd3e2

	@convert_mongoengine_field.register(mongoengine.BooleanField)
	def convert_field_to_boolean(field, registry=None):
	return NonNull(Boolean, description=field.db_field)

Why is this?