Fiora ULT bug

Fiora's ult is not a skill shot, so therefore it should not be able to hit champions that she is unable to see, just like zhonyas stop her ult I would think invisibility should work the same way.

I was laning against a vayne top layne, upon reaching lvl six I engaged and ulted her, vayne ulted and tumbled turning invisible but I continued to ult her despite being unable to see her. Is this intentional or a bug? I think it is pretty broken if it is intentional, because that means she should be able to hit any champ in the area if stealth-ed.