Garen W passive

Garen's W passive 'stacks' whilst there are no points in it - unsure if this is a bug, or intended.

By this I don't mean that you gain armor/MR from it whilst it's not leveled, but if you kill 10cs, then put a point into it, it counts that 10cs you've already got and instantly gives you 10 stacks on the W (so 2.5 armor/MR in this case)

Reproducing? Going into a game, kill some creeps/jungle mobs, put a point into W. See that armor/MR gained from W is equal to 0.25*cs.