I also put this on the discussion forum, thinking there might be something special about the types of essence that fulmination will work with, but I gather now that this is more likely a bug, so I'm repeating it here.
Finally got my mage to level 45 and thought I'd try out fulmination.
When I try to use it, however, I get told that I must have an essence equipped to use this spell.
The thing is, that I do have an essence equipped (life force essence, level 42).
This jpg might help with some details.

