Die during Tryndamere ultimate

So, I am playing Tryndamere against Riven and what happened was I was fighting her. My health was low, so right at the right time I activate ult, she then stuns me. The sound plays when my ult activates and it had not been 5 seconds(it was 2 seconds that I had it activated). Even after, my health drains to 0 and I die. From what I read of the ability, it is 5 seconds activate no exception. This has happened multiple times to me. I originally thought it was just me thinking such, but I counted the seconds and it was about 2 seconds active. Is anyone else having this problem?

EDIT: As far as I know, it only happens when fighting Riven.