In the Mistborn Adventure Game, a person who is a Misting has their Powers level set at Average. However, a Misting who burns Aluminum has an effectively worthless power since it requires the ability to burn other metals to do anything at all. This is why they are called an Aluminum Gnat.
So say for some reason I want to play an Aluminum Misting, whose only power is the ability to burn off any ingested aluminum. Do I set his Powers level at Average and gain a non-existent stunt (there are no stunts for Aluminum), or do I set his Powers level at Weak since there is no practical application for being an Aluminum Misting?