Lucian Ult not doing bonus damage to Minions.

I was playing lucian, and felt like my ult was not doing much damage to minions. so i read the numbers, and it should have been doing 70*4 (for the bonus 400% vs minions). I tested on a wave of minions, with inhibitor bonuses or anything, and it was doing a flat 70 per hit. The damage to champions was unaffected. I only played one game, so I am not sure what steps (if any) are required to reproduce it, other than picking lucian and using his ult on minions.