Using Notifications in Laravel 5.4

I've been trying to implement Notifications into my App, by storing them in the database as detailed in the documentation here: https://laravel.com/docs/5.5/notifications

This is my Truck model with the notifiable trait referenced

<?php

namespace App;

use Illuminate\Database\Eloquent\Model;

use Illuminate\Notifications\Notifiable;

use App\Driver;

class Truck extends Model
{
    use Notifiable;
}

Here is the notification class I use named due_for_maint

<?php

namespace App\Notifications;

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

class due_for_maint extends Notification
{
    use Queueable;

    /**
     * Create a new notification instance.
     *
     * @return void
     */
    public function __construct()
    {
        //
    }

    /**
     * Get the notification's delivery channels.
     *
     * @param  mixed  $notifiable
     * @return array
     */
    public function via($notifiable)
    {
        return ['database'];
    }

    /**
     * Get the mail representation of the notification.
     *
     * @param  mixed  $notifiable
     * @return \Illuminate\Notifications\Messages\MailMessage
     */
    public function toMail($notifiable)
    {
        return (new MailMessage)
                    ->line('The introduction to the notification.')
                    ->action('Notification Action', url('/'))
                    ->line('Thank you for using our application!');
    }

    /**
     * Get the array representation of the notification.
     *
     * @param  mixed  $notifiable
     * @return array
     */
    public function toArray($notifiable)
    {
        return [
          'truck_no' => $this->truck->registrationNo,
          'Milage' => $this->truck->readingMilage,
        ];
    }
}

Here is my a function in my truck controller called notifi_me method which I use to create the notifications

public function notifi_me(){
$truck = Truck::find(10);
$truck->notify(new due_for_maint($truck));



}

Whenever I call the notifi_me function, I keep getting this error:

Undefined property: App\Notifications\due_for_maint::$truck

Could someone explain why this is the case and how I can fix it? My understanding is that Laravel establishes a relationship between the Truck object and the notification, which should make the trucks attributes referencable using syntax like $this->truck->id in the notification class.