factory 生成随机文章时报错。实在是找不到哪里错了。

// 1. 这里是model文件  Article_url.php 的代码
namespace App;

use Illuminate\Database\Eloquent\Model;

class Article_url extends Model
{
    protected $fillable = ['title','body','url_link','user_id','last_user_id'];
}

 //2.以下为  ModelFactory.php 文件内的代码
$factory->define(App\User::class, function (Faker\Generator $faker) {
    return [
        'username' => $faker->username,
        'email' => $faker->email,
        'avatar' =>$faker->imageUrl(256,256),
        'password' => bcrypt(str_random(10)),
        'remember_token' => str_random(10),
    ];
});
$factory->define(App\Article_url::class, function (Faker\Generator $faker) {
    $user_ids = \App\User::lists('id')->toArray();
    return [
        'title' => $faker->sentence,
        'body' => $faker->paragraph,
        'url_link' => $faker->sentence,
        'user_id' => $faker->randomElement($user_ids),
        'last_user_id' => $faker->randomElement($user_ids),
    ];
});

命令行报错提示：