So I don't really know exactly how it's supposed to work, but I finally installed the Arcane Assassin pack and I got my bone charms and the whale statue that gives me an extra slot.

The problem is that Piero doesn't sell an extra bone charm upgrade to match that slot... I have 8 potential bone charm slots, but even after buying all the upgrades from Piero I can still only equip 7 and there's one left that's still greyed out.

How do I enable that slot so I can equip an 8th bone charm? Is this another bug with the game?