icon_frame doesn't work in star_classes?

[Mitchz95]

I've been trying to use icon_frame to get a certain star icon (one of the planets from planet_type_icons), but it just dumps the whole image there instead.

sc_earth = {
   class            = earth_star
   icon           = planet_type_icons
   icon_frame       = 4
   planet           = pc_space
   spawn_odds        = 0
   num_planets       = 0
}

Am I overlooking something incredibly obvious, or does icon_frame just not work in star_classes?