Youtube LiveChat API: messageTextInvalid

I'm using the Youtube livechat.insert API (https://developers.google.com/youtube/v3/live/docs/liveChatMessages/insert), and occasionally get back the messageTextInvalid error. However the error description page doesn't give much of a hint as to what's wrong here (https://developers.google.com/youtube/v3/live/docs/errors#liveChatMessages_youtube.liveChatMessages.insert). I've ensured the messages being sent are <= 200 characters long, and do not have newlines in them. However a message like '(Discord) person: COMMENT : Wonderful!' returns this error that the text is invalid. Is there some information that I can find about what text is valid in the message? I also didn't find much information on the livechat message resource page (https://developers.google.com/youtube/v3/live/docs/liveChatMessages#resource)