Bug for upgrading/constructing Tall Stump and Grumbly Pot

Hello :)

I was just wondering if anyone else has experienced a bug that you can't upgrade the Tall Stump and Grumbly Pot after the Behemoth grows larger, or am I just too dumb to do it? Because upgrading the other two was possible. When I stand in front of it, it just says "Invite". If I let the Behemoth play anyway, he gets stuck, and I have to restart the game.