It is for Mass Curse only. The problem is a capital letter misplaced in the calculation:

       <GameModifier>
            <ModType>Unit</ModType>
            <Attribute>AdjustUnitStat</Attribute>
            <StrVal>UnitStat_Defense_Pierce</StrVal>
            <Duration>3</Duration>
            <Calculate InternalName="Calc" ValueOwner="TargetUnit">
                <Expression><![CDATA[[UnitStat_Defense_PIerce] * -1]]></Expression>
            </Calculate>
            <Calculate InternalName="Value">
                <Expression><![CDATA[[Calc]]]></Expression>
            </Calculate>
        </GameModifier>

Thanks for tracking it down. I've updated the post title.