Prophet quest glitch - keystone not appearing on corpse

when you try and free the moth priest there is supposed to be a keystone to deactivate the barrier around the priest and free him. then you fight blah blah blah. I played through before on the dawnguard side and there were no glitches but as a vampire the keystone isn't on that vampires corpse. does anyone know how to fix it or do i need to start over with another character?

double checked the chest and its not in there and the quest marker is indicating that its on the corpse and thats where it was when I played through on the dawnguard side