GP ult items not purchasable

I was just playing GP in a team builder match and when I got to 500 silver serpent tokens I could not spend them. I had exactly 500 so I thought that I had to have more than exactly 500 so I went and killed a few jungle camps and got a few more coins. I went back to the shop and still could not buy a single one of the new items for his ult. I left the fountain and joined my team for team fights and died and still could not buy the items.