Laravel 5.7 - Markdown Notification未声明变量异常

use Illuminate\Bus\Queueable;
use Illuminate\Contracts\Queue\ShouldQueue;
use Illuminate\Notifications\Messages\MailMessage;
use Illuminate\Notifications\Notification;

class ExampleNotification extends Notification
{
    use Queueable;

    protected $variable1;
    protected $variable2;

    public function __construct($variable1, $variable2)
    {
        $this->variable1 = $variable1;
        $this->variable2 = $variable2;
    }

    // ...
}

$variable1 = 'Value 1';
$variable2 = 'Value 2';

$user->notify((new ExampleNotification($variable1, $variable2))->with([
    'variable1' => $variable1,
    'variable2' => $variable2,
]));

public function toMail($notifiable)
{
    return (new MailMessage)
        ->line('Hello!')
        ->line('Variable 1: '.$this->variable1)
        ->line('Variable 2: '.$this->variable2);
}