When spending a buff as a cost, do you target a unit, and if so, is an unbuffed unit an illegal target?

Ruling: Costs are not targeted. When spending a buff as a cost, you do not target a unit.

Nuances:

• If you try to buff a unit that already has a buff, you do target them, but you won't be able to buff them for the purposes of effects like "when you buff a unit" (unless they have special exceptions like Lee Sin, Ascetic)