Forum:Worm Apocalypse Flip Effect Target

Ok...I have a question about the target from "Worm Apocalypse's" effect.

When "Worm Apocalypse" is flipped over it destroys one spell or trap card on the field. Lets say my opponent plays a face down "Worm Apocalypse" and i trigger it, however i have no targets for the effect, while my opponent has several face down spells/traps. Does my opponent have to destroy one of his own spells/traps? or does the effect just not do anything?