Accepting AI Open Borders Agreements causes game to hang indefinitely

[Ramesses]

As the title says, if an AI offers me a two-way Open Borders agreement and I accept, the game has a 25%-50% chance of hanging indefinitely. The diplomacy panel and buttons all disappear, while the leaderhead just animates forever. I play in windowed mode, so I can just hit the X on the game window to exit out (and the "do you want to exit" dialog box even pops up to confirm it), but gameplay is essentially halted, and I have to exit the game and resume from an earlier save.