Can hold Nautilus Shell in offhand slot

The bug:
I can hold a Nautilus Shell in my offhand, which is most likely not supposed to happen.

What I expected to happen:
I expected not being able to hold the Nautilus Shell in my offhand slot.

Steps to reproduce:

1. Place a Nautilus Shell item in your offhand slot.
    Notice it lets you put the item in your offhand slot.