Tourism boost via Open Borders bug?

Hello,

I'm on the verge of a cultural victory, with just one remaining civ to become influential over (Assyria).

For all the other civs, when I hover over the Tourism score I see a message which says something along the lines of "25% extra for Open Borders", but for Assyria it say "0% extra for open borders", even though I do definitely have Open Borders with them.

Is this a bug, or does Assyria have some special ability in this regard?

Thanks,
Ackoman