golang http.ResponseWriter panic

err:
panic: runtime error: invalid memory address or nil pointer dereference

用&http.Client{}发：

    msg_req, err := http.NewRequest("POST", sendMessageUrl, bytes.NewBuffer([]byte(postStatus)))
    if err != nil {
        log.Println(err)
    }
    msg_req.Header.Set("Content-Type", "application/json")
    client := &http.Client{}
    _, err = client.Do(msg_req)
    if err != nil {
        panic(err)
    }
    defer msg_req.Body.Close()