Cant attack nexus in CoOp

Sorry I don't have a video of this, but I hope you believe me.

We were about to win a bot game, both nexus turrets were down, when suddenly their nexus became untargetable. The red sword with the circle and line through it appeared. It's health bar disappeared, and minions couldn't attack it either. This happened for about 3 minutes. I recalled, then went back. A minute after, its health bar reappeared, and minions quickly finished it off.

Keep in mind this was Coop vs AI. I was playing Riven, dont know if that makes a difference or not.