In a recent game a spellcaster cast curse of the putrid husk at a fire elemental.
The spell reads
Illusion (Phantasm) [Fear, Mind-Affecting, Evil]
Level: Brd 3, Sor/Wiz 3
Components: V, S, M
Casting Time: 1 action
Range: Close (25 ft. + 5 ft./2 levels)
Target: One creature
Duration: 1 round + 1d10 minutes
Saving Throw: Will negates
Spell Resistance: Yes
This illusion forces the subject to believe his flesh is rotting and falling off his body, and that his internal organs are spilling out. If the target fails his saving throw, he is dazed (and horrified) for 1 round. On the following round, he falls unconscious for 1d10 minutes, during which time he cannot be roused normally.
Now thinking of a fire elemental it doesn't have organs or flesh as it is comprised of fire, but reading the fire and extraplanar subtypes and the elemental stat black it was ruled that the spell technically still worked.
Is this just something that is mostly up to the DM or are we missing a rule somewhere?