Spell that targets creatures cast against a goblin. But goblin is illusion. Spellslot wasted?

Spell that targets creatures cast against a goblin. But goblin is illusion. Spellslot wasted? Perhaps – I could see a generous DM ruling that if the spell targets a creature, the slot is not expended. I'd say action wasted, though

— Mike Mearls (@mikemearls) October 12, 2017