Laravel 5.4 - 没有为[mail]定义提示路径

Whenever a user is being registered, this code is being dispatched:

\Mail::to($user)->queue(new UserRegisteredMail($user));

The build method in UserRegisteredMail uses the markdown as you can see here:

/**
 * Build the message.
 *
 * @return $this
 */
public function build()
{
    return $this->markdown('emails.user.registered');
}

The difference between this question to No hint path defined for [mail] Laravel 5.4 is that I'm already using the markdown method and I'm still getting this error. Any ideas?