Illusion shouldn't be able to disguise the Pokémon as a fainted Pokémon

I was playing a Randomizer, and after Zenith's Rayquaza fainted, he sent in his Zoroark, which disguised itself as Rayquaza (his only other living 'mon was Arbok). Illusion should only disguise the Pokémon as a conscious Pokémon, rather than being able to disguise it as a fainted Pokémon