[Xerath] BUG REPORT - Different Ultimate Bug

I've played Xerath now a few times (3 or 4). The first time I played him, there were no issues. Then starting on the 2nd game, I encountered a bug with his ultimate. I used it twice in that game. The first time I used it, it worked fine. Starting on the 2nd time, whenever I hit the ultimate, Xerath starts channeling. The targeting circle appears as well as the range indicator. However I can't fire. No matter where I click, he won't fire any shots. I've tried this with smartcast on and off. I've tried letting the channel duration expire and then trying again when the CD is off. I've tried cancelling and retrying later. Nothing works. Unfortunately, I do not know what caused this to happen. Now it happens every game, so I can't even attempt to reproduce how it started.