功能完成，状态为“无法与功能通信”

I have a simple go google cloud fn that does some calculation and then reports certain data, say X, to my slack API which is supposed to send a message with X to a slack channel.

I tested X on postman, and slack did send a message successfully. However, in my cloud function, right at where http.Post is invoked, the function stops responding, and logs read 'function finished with status: cannot communicate with the function'.

func sendMessage(domains []BadDomain) (*http.Response, error) {
        requestBody, err := prepareBody(domains)
        if err != nil {
                log.Fatal("Error preparing the request body --- ", err)
                return nil, err
        } else {
                resp, err := http.Post(url, "application/json", requestBody)
                if err != nil {
                        return nil, err
                } else {
                        return resp, nil
                }
        }
}

requestBody is io.Reader. The json payload is correct, I have tested it via postman.