Can't ask to convert?

[Arminius]

For some reason in my current game, I've been asked to convert several times (not unusual). However, I finally decided to ask someone to convert. Unfortunately, I cannot ask any Civ to convert to Taoism, even Greece who has 7 Taoist cities and 1 Christian (not Holy City of Christianity, either). Does anyone know why I can't ask anyone to convert?