无法在控制器laravel5.6的子文件夹中获取当前登录的用户ID

Question

无法在控制器laravel5.6的子文件夹中获取当前登录的用户ID

已添加控制器中

use Illuminate\Support\Facades\Auth;

试图通过使用Auth::user()->user_id;来获取，但返回null

namespace App\Http\Controllers\API;

use Illuminate\Http\Request;
use App\Http\Controllers\Controller;
use App\User;
use Illuminate\Support\Facades\Auth;

class IndexController extends Controller
{
    public function index(){
        return Auth::user()->id;
    }
}