Can't send my tribute to the emperor during third (peaceful) city

[Former user]

I've saved up my 10 oil to send to Caesar during the third city/assignment, yet can't send it! I was notified by my advisor, clicked the send tribute button, and nothing happened! The button still says I have 17 months to pay up (was a little late, but have more than enough oil now), and the message near the cursor says to 'click to send tribute'. The 'send tribute' button doesn't outline in red like the surrounding 'send a gift to emperor' buttons when I go to click it either.