**Invalid Targets.** If you cast a spell on someone or something that can't be affected by it, nothing happens to that target, but if you used a spell slot to cast the spell, the slot is still expended. **Do You Know.** If the spell normally has no effect on a target that succeeds on a saving throw, the invalid target appears to have succeeded on its saving throw, even though it didn't attempt one, but the caster is aware the target was an invalid target. Otherwise, you perceive that the spell did nothing to the target.