Does fondling one's wife during the six days of Shawwal fasting, which resulted in the emission of semen without intercourse, invalidate the fast?

The fasting of one observing a supererogatory fast is invalidated by the emission of semen through direct contact or foreplay. However, there is no sin upon him for this, because "one observing a supererogatory fast is the master of his own self; if he wills, he fasts, and if he wills, he breaks his fast." Nevertheless, it is recommended for him to make up that day.