Is scratching the penis, which leads to the unintentional emission of semen, considered breaking an oath sworn on the Quran to abandon masturbation, and what is its expiation?

Whoever swears by the Qur'an to abandon masturbation, then rubs his penis until semen is discharged, is liable for an expiation for breaking an oath, even if he did not intend the discharge; this is because rubbing the penis is a prelude to masturbation.