Invalidation of fast

Salaam I have an important question in regards to invalidation of fasting. I was keeping an optional Shawwal fast and was going to do sunnah Jummah Ghusal. Before Ghusal I was shaving my genital area for cleanliness in the process I felt slight arousal but carried on and unexpectedly ejaculated it was very quick and I didn't to ejaculate that fast as I wasn't even fully aroused but slightly. I had nafil fast and didn't intentionally masturbate although I was aroused during the shaving off my pubic. Is it fardh for me to make up the fast? JazakAllah Khair