Model serializer without any fields

Question

I would like to have serializer which only validate instance field "default" and return nothing. I want do that without picking any fields but I'm receving an error that i need to pick some fields or use all_, can you please help me what i can do then ?

class PaymentMethodDefaultSerializer(serializers.ModelSerializer):

class Meta:
    model = PaymentMethod

def save(self, **kwargs):
    instance = PaymentMethod.set_as_default(self.instance)
    return instance

@staticmethod
def validate_default(value):
    if value:
        raise serializers.ValidationError(
            'There is an default payment'
        )
    return value