Many powers and other effects grant the ability to deal extra damage. Extra damage is always in addition to other damage and is of the same type or types as that damage, unless otherwise noted. Because of this rule, an effect that deals no damage cannot deal extra damage. However, a power doesn’t necessarily have to hit a target to deal extra damage—it needs only to deal damage to the target.
Example: Valenae the cleric might have an ability that causes her to deal 5 extra radiant damage to undead creatures. That ability means she deals 5 extra radiant damage whenever she deals damage to an undead creature.
Published in Monster Manual 2, page(s) 217, Player's Handbook 3, page(s) 220, Rules Compendium, page(s) 223, Monster Vault: Threats to the Nentir Vale, page(s) 123.