Trying for the witch's 6th quest; Magma mires as a priest without potions.
I had been worshipping taurog.
The dungeon has three bosses, I had just killed the magic immune boss with two attacks (death protection for the second) and then I hovered over the third boss (who I am not able to fight yet) and got the exception.
Possibly of note: I am still at 1 health, have a small fine sword, have converted a lot of stuff, and have a full inventory.