Not performing a rukn invalidates nāfilah prayers; however, performing an additional rukn does not invalidate them. Therefore, if one forgets one of the acts of nāfilah prayers and remembers it when he has started to perform the next rukn, he must perform the act and then perform the rukn again. For example, if while performing rukūʿ he remembers that he has not recited Sūrat al-Ḥamd, he must go back and recite Sūrat al-Ḥamd and then perform rukūʿ again.