The guards keep killing my incubus before I can

I'm an undead warlock, and I got 2 quests to get some people's hearts and then summon a succubus and an incubus. I summoned and killed the succubus just fine, but when I summoned the incubus, a guard comes and kills it. It doesn't count towards the quest. I have waited until the guard is gone, but when I summon it, a guard comes out of nowhere. Pls help.