Brand ult uses mana before it fires

A couple different times, I've noticed that 's ult can be cancelled if he is CCed at just the right time. It looks like his ult uses up his mana, but if he gets stunned right after that but before the fireball comes out, the fireball simply vanishes. Since his mana has been used, he has to try to work it up from the beginning. If he has , since he didn't "cast" his ult, it has no effect, even though he worked his mana up to max and then drained it.

I strongly urge that either his mana is used WHEN his fireball comes out.