Can't stop channeling Absolute Zero

Ok I am not a Nunu player at all, but from around 25 times I have played with him in all these years I am almost confident that his ulti can be cancelled at any time right? Well I just played a game where I couldnt stop channeling untill the full duration and everyone walked out of it every single time. Am I having an Alzheimers here or is Nunu bugged this patch ?