Zoho Flow Slack error, "invalid_thread_ts"

I've created a Flow and want it to send a private channel message on success or failure of the actions. In the past I haven't been getting error messages when the Slack stage failed. It just didn't do anything--it wouldn't send the message and wouldn't throw an error. Today, for some reason, I'm finally getting an error message, Slack says "invalid_thread_ts". I've looked it up and, according to Slack this is a thread error.

I need some help debugging this. It doesn't make sense that I would have this error because I am not doing anything with a thread. I'm using the "Send private channel message" component. It doesn't have any options in the form to interact with threads, and I don't need to. I just need to send a message to a private channel.