Elise spider vs Karthus - too op?

I was just in an aram game as elise. I was vs a karthus on the enemy team and decided to use my instincts when karthus ulted : hop into spider form and E to jump. Surprisingly...it dodges karth ult. I doubt this was intended as any other damage (ie: ignite, malzahar e) still take into effect. Was this intended? I was shocked the first time and made sure i tested it 2 more times. I was able to dodge 3 out of 3 ults that he cast, so it is confirmed : it does in fact completely dodge a karthus ult. I had a few people argue that it does not dodge the damage, but i can personally confirm it does. If this was intended by the mechanics, i think it makes elise much too op and karthus much too weak in that respective manner. I think if Karthus has to have a 5-6 minute cooldown, it shouldnt be able to be trumped by a simple 14 second cd.

Just wanted to put that out there in case it was not meant to be.