Urgot Ult Cancelling

Last night in a Normal Draft game I tried on separate occasions to Ult Lux using Urgot, but on at least two attempts the ult just cancelled on its own and left me with the cooldown timer. In one of the attempts I tried to ult Lux over the border of the Baron pit. I was also able to get it to occur again in a custom bots game today. I think it has about a 15% reproduction rate.

1. Get in Range of Lux
2. Cast Urgot Ult on Lux

I don't know if this is specific to Lux, but she's the only person I've noticed this issue with.