Nunu Ult. not dealing damage

Experienced this a few times in the passed 3 days. I'll run up in the middle of a team and a creep wave i'll ult, and if they break it with CC no matter how long the channel, It will deal 0 damage. The minions wont even take any damage... It happened twice in 5.2 and once this morning in 5.3. Also i went bot lane while my team was top to grab a double siege wave... started channel (almost full channel) and out pops a nami bubble from the bush and i did 0 damage to the creep wave again..