sylphide

I've recent obtained an Incubus pet, and I've been trying to equip pet accessory Masked Ball onto it. However, it will not equip, and I did double check that it is the correct accessory for the pet. I've also tried "unequipping" despite the item never having been equipped, and that did not work either. To my knowledge, double-clicking the item should equip it to the pet, but that doesn't seem to be the case here? Any and all help would be greatly appreciated, so thank you in advance! C: