Benevolence to parents is Wajib even if both of them are ‘Mushrik’

"A letter on various aspects of Islamic Shari’a, written by Imam al-Ridha (A) to Mamun, includes the following: “Benevolence to parents is Wajib even if both of them are ‘Mushrik’. However obedience to parents is not Wajib if their order is against the orders of the Creator.””