It's not fearsome with Titanium Shields, since you can always destroy them.
It's OP with Hope, Emerald Shield and Reflective Shield, because there is no way to destroy them.
Maybe you can make it "Target shield damage reduction is increased by 1. If your mark is
, increase your shield damage reduction by 1."
I know the wording is awful, but it's only to say that you must target a shield if you haven't the
mark. If you have
mark it increase defense without targeting, so even immaterial shields can be enhanced.
But my wording is awful, i know!