[TFS 1.3] What could be wrong with this onHealthChange?

function onHealthChange(creature, attacker, primaryDamage, primaryType, secondaryDamage, secondaryType, origin)
    if attacker then
        if creature:getOutfit() == 9 then
            primaryDamage = -primaryDamage
            secondaryDamage = -secondaryDamage
        end
    end
    return primaryDamage, primaryType, secondaryDamage, secondaryType
end