No Valid Assasination Targets?

This has to be a bug, right?

I have one of my spies running around. He reveals a gallic spy. I bring up an assassin, and try to assassinate the gallic spy. Right click, menu opens, but says there is no one to assassinate.

Now, I *swear* I've killed spies and diplomats like this before a lot - why does it suddenly not work? (I even ended the turn and brought up a different assassin to see if it was any different).

ive met unbribable diplomnats...