Unknown glyph found in class

I am customising an open source font. I have deleted some glyphs so I can concentrate on a small amount of characters. When I try to export the font I get the error message "Unknown glyph " followed by the name of one of the glyphs I deleted “Found in class aalt”
I’ve tried googling for classes in glyphs. I have a feeling they refer to OpenType features but I don’t understand what a class is. I clicked the “show” button and it brings up the Features page but I’m not sure what I’m looking at.

Have a look in Font Info > Features. If you don’t a red error marker, hit the Compile button in the lower left.