Goodbye energy shield

I just traded for an amulet that should be increasing my energy shield. Instead it removes it completely. I gave it to a guild member to try out, it works fine for him
Why is this happening?

So I tested something. If I take eldritch battery off. I lose some energy shield from the amulet change, but keep my energy shield. As soon as eldritch battery comes back on, it drops my energy shield to 0.

I'm going to assume that you have swapped from The Everlasting Gaze to another amulet, if so Eldritch battery will take your ES to 0 if you have it on (right now Everlasting Gaze gives a portion of mana as ES, not sure if it's bugged or intended). If you swap it to another amulet, you'll have no ES with Eldritch Battery on. If you are using Chaos Inoculation, then you will have to resort to Mind over Matter which will make your Mana become your HP pool.

As these people have mentioned already. Everlasting Gaze amulet gives you energy shield AFTER Eldritch Battery. For some reason it doesnt consume the Energy Shield it otherwise gives you. Youre just losing the ES that EG gives you.