Can't buy a champion with enough ammount of IP.

I was on my way to buy Anivia (3150 IP), having 4388 IP, but the buy button is grayed out, and it says I dont have enough RP/IP. I've tried re-logging but it doesn't change anything. It's the same problem with all other champions costing 3150 or more. I'd add a screenshot if I could but I don't know if its possible or if it's just really hard to figure out how to do it. Help me.