Darius Q + Urgot R weird interaction/bug

You can see that my Q was on him and it should have hit him. He did not use his spellshield (I checked the replay). I'm not sure if Urgot's ultimate cancels casts, but I highly doubt that. I'm going to say that it's probably caused by the untargetability it gives you or the supression, but I don't think it should work like this.