Nasus's Q Bonus damage - Doesn't crit?

So, in the recent buff given to vayne's Q - tumble, rioters said that either both parts of her attack crit, or none of them do. I was wondering if that applied to Nasus's Q as well considering it's used as an active spell, and as an enhanced auto attack, why don't both parts of the spell crit? It feels pretty bad to not only not be able to teamfight effectively, but also play against champions like GP who can not only erase your wither, but also send an on-hit crit with their ranged Q poke. Thoughts?