What happens when your single-target spell has an inappropriate target?

[Natural Six]

You have a spell that specifies humanoid targets, and you announce that you're going to cast it on a dog. Per RAW under Ranges, Areas and Targets (p. 196) ". . . your spell fails to target that creature." Rules specify that a multiple target spell can affect the rest of the targets. What does this mean for a single-target spell?

A: Sorry, you can't cast that spell on a dog. Try something else.
B: Your spell fails because your target can't be a dog.

Has anyone found clarity on this?

[DM_Blake]

Seems fairly clear. The answer is B, but only for area spells.

As per the rules you mention:
.

If you cast an area spell on 3 targets and 1 of them is invalid, you DO cast your spell and it affects 2 of the targets and ignores 1 target.

If you cast an area spell on 2 targets and 1 of them is invalid, you DO cast your spell and it affects 1 of the targets and ignores 1 target.

If you cast an area spell on 1 targets and 1 of them is invalid, you DO cast your spell and it affects 0 of the targets and ignores 1 target.

However, I apply that reasoning only on area spells. On spells that you specifically must select targets, I just don't allow the invalid target to be selected and if no target is selected/valid, then the spell cannot be cast.

I think this is a bit of a gray area but the idea is this:

If a spell requires a target and you don't have a valid target, then you have not met the requirements for casting it and therefore cannot cast it (which also means you don't fail to cast it and don't lose the spell since you are not actually casting it).

Targeted spells require a target. Area spells do not.

[Natural Six]

Thank you; I was leaning that way myself. I guess the further concern is that certain spells could give away information for free if adjudicated this way. If the spell specifies "living target" and you try to target a vampire you haven't identified as one, then you've found out your target isn't living. Or something else wonky is afoot. Such is the gaming life.