Lux Ult > Bard Ult

I was playing a game earlier as Bard, while in a teamfight I ulted onto Lux right as she started her ult and instead of it stopping her she was able to complete the cast and her beam killed 2 of my members. Now I know you're gonna say maybe I didn't hit her before it shot but it most definitely did. It hit her pretty much as soon as her red indicator appeared. Is this supposed to happen? I mean it should have interrupted it if I'm not mistaken right?