ProjectFlyer Photo upload issues

Hi,

I've just followed the video "Bulk File Uploads" but I'm getting the following error when uploading a file.

FatalErrorException in FlyersController.php line 44:
Call to a member function addPhoto() on null
    in FlyersController.php line 44

My FlyersController.php

<?php

namespace App\Http\Controllers;

use Illuminate\Http\Request;

use App\Http\Requests\FlyerRequest;
use App\Http\Controllers\Controller;

use App\Flyer;
use App\Photo;

class FlyersController extends Controller
{
    public function create()
    {
        return view('flyers.create');
    }

    public function store(FlyerRequest $request) 
    {
        Flyer::create($request->all());

        flash()->success('Success!', 'Your flyer has been created.');

        return redirect()->back();
    }

    public function show($zip, $street)
    {      
        $flyer = Flyer::locatedAt($zip, $street);

        return view('flyers.show', compact('flyer'));
    }

    public function addPhoto($zip, $street, Request $request)
    {
        $this->validate($request, [
            'photo' => 'required|mimes:jpg,jpeg,png,bmp,gif'
        ]);

        $photo = Photo::fromForm($request->file('photo'));

        Flyer::locatedAt($zip, $street)->addPhoto($photo);
    }
}

My Flyer.php Model

<?php

namespace App;

use Illuminate\Database\Eloquent\Model;

class Flyer extends Model
{
  protected $fillable = [
  'street',
  'city',
  'state',
  'country',
  'zip',
  'price',
  'description'

  ];

  public static function locatedAt($zip, $street)
  {
    $street = str_replace('-', ' ', $street);

    return static::where(compact('zip', 'street'))->first();
  }

  public function getPriceAttribute($price)
  {
    return '$' . number_format($price);
  }

  public function addPhoto(Photo $photo)
  {
    return $this->photos()->save($photo);
  }

  public function photos()
  {
    return $this->hasMany('App\Photo');
  }
}