laravel5.5使用permission出现Column'model_id'cannotbenull?

SQLSTATE[23000]: Integrity constraint violation: 1048 Column 'model_id' cannot be null (SQL: insert into `ks_model_has_permissions` (`model_id`, `model_type`, `permission_id`) values (, App\User, 2))

$user->givePermissionTo('add articles');

namespace App;

use Illuminate\Notifications\Notifiable;

use Illuminate\Foundation\Auth\User as Authenticatable;

use Spatie\Permission\Traits\HasRoles;

class User extends Authenticatable

{

    use Notifiable;

    use HasRoles;

    protected $fillable = [

        'name', 'email', 'password',

    ];

    protected $hidden = [

        'password', 'remember_token',

    ];

}

namespace App\Http\Controllers;

use Illuminate\Http\Request;

use Spatie\Permission\Models\Role;

use Spatie\Permission\Models\Permission;

use app\User;

class HomeController extends Controller

{

    public function __construct()

    {

        $this->middleware('auth');

    }

    public function index(User $user)

    {

             $user->givePermissionTo('add articles');

    }

}