Fireball and some other spells are not friendly fire locked, Burning Hands is. The question has come up several times already since the Beta of Fallen Enchantress, as long as there is anything in the targeted three tiles other than an enemy unit or nothing at all, you cannot cast the spell.
Ah, got it. But if that is the case, and if people keep mistaking this for a bug, that means that the player needs better feedback. Instead of the tool tip saying "Need to target an enemy unit", it should say "friendly units not permitted in area of effect".
Thanks for clearing this up!