Auth::user() return null

I use Laravel 5.2 and have a problem with middleware. There is the code in the routes.php

    use Illuminate\Contracts\Auth\Access\Gate;


    Route::group(['middleware' => 'web'], function () {

        Route::auth();

        Route::get('/', 'HomeController@index');
    });


    Route::group(['prefix'=>'admin',  'middleware' => 'admin'], function(){
        Route::get('/', function(){
            return view('admin.index');
        });
        Route::get('/user', function(){
            return view('admin.user');
        });
    });

Kernel.php:

    protected $routeMiddleware = [
    ...
     'admin' => \App\Http\Middleware\AdminPanel::class,
    ];

AdminPanel.php

    namespace App\Http\Middleware;


    use Closure;
    use Illuminate\Support\Facades\Auth;
    use App\Role;

    class AdminPanel
    {
        public function handle($request, Closure $next)
        {
            $user = Auth::user();
            dd($user);

            if($user){
                $role = Role::whereName('admin')->first();
                if($user->hasRole($role)){
                    return $next($request);
                }
            }
            return redirect('/');
        }

So,

$user = Auth::user()

always return null. Thanks for suggestions!

via Chebli Mohamed