"You cannot send messages to users who are not following you" error after starting DM conversation


#1

We are running DM Chatbot on an account that allows “Receive Direct Messages from anyone”.
When the chatbot receives a message event from a user and send a message back to the user with POST direct_messages/events/new, we sometimes receive “You cannot send messages to users who are not following you” error.
I believe we should not get the error in this case, but what causes the error?