whimsy not working on fiddle after ulti, possible bug?

I was in an aram game playing as lulu and there was a fiddle on the enemy team. I encountered a problem with my whimsy when I tried to use it on fiddle after he ult-d. I tried to interrupt his ulti more than once using whimsy and every time fiddle remained un-fuzzied and still very murder-y. my whimsy would work normally to interrupt his drain or when chasing but for some reason did not seem to work post teleport during his ulti. Is this a bug or does he become un-interruptable at that point?

any insight would be greatly appreciated, thanks in advance. -regal