laravel 6.x 全文搜索Scout生成宏命令出错

根据文档配置count宏命令
出现以下错误

以下是我的代码

<?php

namespace App\Providers;

use Illuminate\Support\ServiceProvider;
use Laravel\Scout\Builder;

class AppServiceProvider extends ServiceProvider
{
    /**
     * Register any application services.
     *
     * @return void
     */
    public function register()
    {
        //
    }

    /**
     * Bootstrap any application services.
     *
     * @return void
     */
    public function boot()
    {
        Builder::macro('count', function () {
            return $this->engine->getTotalCount(
                $this->engine()->search($this)
            );
        });
    }
}

请问这种怎么解决呀？