Is something special needed at the end of the "Stadium" level to get the "Touchdown" challenge besides having twenty zombies "alive" at the end of the level? I've had 52 still "alive" when the clock counts down to zero, and the challenge doesn't trigger as complete.

Any insight would be appreciated.

EDITED TO ADD: Never mind. Unlike other levels, you need to actually click on "Finish" before the level acknowledges that you've also met the challenge conditions.