def refund_order(self, order_id, **params):
        """https://developers.coinbase.com/api/v2#refund-an-order"""
        for required in ['currency']:
            if required not in params:
                raise ValueError("Missing required parameter: %s" % required)
        response = self._post('v2', 'orders', order_id, 'refund', data=params)
        return self._make_api_object(response, Order)