Potential Bug?

So I've noticed something today and I'm not sure if this counts as a bug or if it was intended or just a minor oversight that is in need of correcting. At any rate the "bug" that I've discovered is that when using Fiora against a Fizz you can't Riposte his ult. You can Riposte the knock up but you can't riposte the initial hit from the fish which in turn procs his ult's DFG effect.

Surely my opinion on the matter isn't of import but when your only options are to instinctively try to riposte and block his fish but then be punished because it doesn't work, or to get hit and DFG'd then Riposte the ult's damage (if you survive the empowered burst damage).

I tried this a few times in game with a friend and it didn't work any of the times we tried it. Seeing as the ult slows you and is a DFG I felt it must've been a bug or an oversight since Riposte counters any CC effects within that window. It's a pretty small issue that probably wont be encountered much since it's only those specific champs but I feel that for the few times it does happen, say in a ranked game, it would be unfair to the poor soul who's skill didn't work as intended.

I could somehow be wrong in this, perhaps bad timing with the Riposte, so if I am just correct me. However, if I'm right this should be fixed to avoid unfortunate losses due to a bug.