code: func checkSender(sender *Sender) error {
  	if sender.ApiKey == "" {
  		return errors.New("the sender's API key must not be empty")
  	}
  	if sender.Http == nil {
  		sender.Http = new(http.Client)
  	}
  	return nil
  }
docs: // checkSender returns an error if the sender is not well-formed and
  // initializes a zeroed http.Client if one has not been provided.