"Call to a member function rating() on null"

[tylerpixelguild]

I went through the installation instruction and I get this error:

"Call to a member function rating() on null"

When executing this code:

namespace App\Http\Controllers;

use Illuminate\Http\Request;
use AmazonProduct;
use DB;
use App\User as User;
use App\Post as Post;

class HomeController extends Controller
{
    public function index()
    {		
		$user = User::first();
		$post = Post::first();

if(\Auth::check()){
		$rating = $post->rating([
  		  'title' => 'Some title',
   		  'body' => 'Some body',
 	      'rating' => 5,
		], $user);

		dd($rating);
}
    }
}