def validate(self, value, model_instance):
		invalid_values = []
		for val in value:
			try:
				validate_slug(val)
			except ValidationError:
				invalid_values.append(val)
		
		if invalid_values:
			# should really make a custom message.
			raise ValidationError(invalid_values % self.error_messages['invalid_choice'])
dataset/ETHPy150Open ithinksw/philo/philo/models/fields/__init__.py/SlugMultipleChoiceField.validate/swapped operands of `%`