Conquering final capital through trade

I ran into this last night and haven't tried reproducing it, and nobody reported it here(that I could find). I was busy cutting down my enemies across the globe and was closing in on the final civ-the Songhai. Marching through their territory, I took their capitol, Gao, but they still had control of the original Mongol capitol. Naturally, I was pleasantly surprised on the next turn when he offered me Karakorum in exchange for a peace treaty. I graciously accepted his complete forfeiture of the game, but as soon as I accepted the trade the dialogue window went away and Askia just kept glowering at me and the game locked up.