BUG: Assassination ideology choice event uses unmet empire

The other day I had one of those ideology choice events with an empire saying that they would declare war on me because according to their spies, I assassinated their ambassador. Okay, well, uh, which ambassador would that be and hi, how are you, nice to make first contact with you.

In other words, I had never even met this empire, and after this event I still hadn't. Checked Diplomacy screen, nope, nobody new there.