Champ select karthus team builder bug?

So I purchased karth with IP, go to team builder solo que. I set up rune/masteries, Karth isn't there, I exit to main screen, try again, no karth. I log out and back in, he's there, I get set up, pop up "Error: Invalid champ selection." I try 5-6 more time to no avail. What's up with that?