Circle of Protection: White
Fourth Edition — Common
Circle of Protection: White rulings:
10/4/2004 : A source of damage is a permanent, a spell on the stack (including one that creates a permanent), or any object referred to by an object on the stack. A source doesn’t need to be capable of dealing damage to be a legal choice.
10/4/2004 : Can be used even when there is no damage to prevent. It prevents the next damage (if any) from the source this turn.