How does Oklaine's Lantern Work?

For the sake of Oklaine’s Lantern, what qualifies as a caster off hand? I recently invested in the damage boost, but it seems to have done nothing to my DPS.

It says it requires a scepter, dagger, or caster off hand. I looked it up, and it appears any off hand (except shields ofc) should qualify.

So I figured using a pistol with an off hand I should be able to get the bonus, but it appears not.

What am I missing :confused: