If the target spell is an illegal target when Countermand tries to resolve, Countermand will be countered and none of its effects will happen. The target’s controller won’t put any cards from the top of his or her library into his or her graveyard.

You may target a spell that can’t be countered with Countermand. If you do, when Countermand resolves, the target spell won’t be countered, but its controller will still put the top four cards of his or her library into his or her graveyard.