Проблема с миграцией в Laravel

При миграции происходит ошибка, не понимаю в чём проблема.
код:
<?php

use Illuminate\Database\Migrations\Migration;
use Illuminate\Database\Schema\Blueprint;
use Illuminate\Support\Facades\Schema;

class ChangeTimestampPosts extends Migration
{
/**
* Run the migrations.
*
* @return void
*/
public function up()
{
Schema::table('posts', function (Blueprint $table) {
DB::statement("ALTER TABLE 'posts' CHANGE 'created_at' 'created_at' TIMESTAMP NULL DEFAULT CURRENT_TIMESTAMP, CHANGE 'updated_at' 'updated_at' TIMESTAMP NULL DEFAULT CURRENT_TIMESTAMP");
});
}

/**
* Reverse the migrations.
*
* @return void
*/
public function down()
{
Schema::table('posts', function (Blueprint $table) {
DB::statement("ALTER TABLE 'posts' CHANGE 'created_at' 'created_at' TIMESTAMP NULL, CHANGE 'updated_at' 'updated_at' TIMESTAMP NULL");
});
}
}
?>