laravel-5.5 使用 permission 出现 Column 'model_id' cannot be null?

错误信息

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))

调下面这句出现的错误

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

详细代码片段

app\User.php

<?php
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',
    ];
}

app\Http\Controllers\HomeController.php

<?php
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');
    }
}