ToME: the Tales of Maj'Eyal

So I made an Adventurer, and decided to pick up Mutated Hand because it sounded cool. Well, the tooltip once I picked it up says that it is not active because my offhand is not empty.

I have tried 2 different characters (of 2 different races), confirmed that I only have a 1-handed weapon equipped without a second weapon or shield, tried deactivating all my sustain spells, and tried removing my gloves.

Please help!