Could not purchase 3rd karthus for upgrade

I was rolling to get a 3rd karthus for a match. I rolled and found the karthus, but could not purchase him. I was able to purchase the other characters available on the roll and sell them back, but I could not purchase the karthus.

This was in the late game, dragon was already done. I had max level 9 experience, and I also had the item that gives +1 team size. I had all of my units on the field that I could, including the karthus i was attempting to upgrade. I was able to upgrade other units to 2-star that were on the field, but I wasn't able to buy the karthus card for the upgrade.