API Reply to the last conversation is failing - user not found

Hi, this is my first post, so go easy :)

I'd like to reply to the last conversation using the api. The URL I'm calling is: 

https://api.intercom.io/conversations/last/reply/p>

I'm using this as reference - https://developers.intercom.com/intercom-api-reference/reference#reply-to-a-conversation/p>

The message body looks approximately like this:

{

  "message_type": "comment",

  "type": "user",

  "user_id": "usersemail@aferry.com",

  "body": "Test message"

}

I've confirmed this user exists but I get this response:

{

    "type": "error.list",

    "request_id": "000sdhmlsb3j8gk6qieg",

    "errors": [

        {

            "code": "not_found",

            "message": "User Not Found"

        }

    ]

}

Any ideas appreciated. I can't see what could be wrong.