mysql 代码给我的外键格式不正确

我想创建一个小型数据库，但每次都收到错误:

下面是MySQL代码:

DROP TABLE IF EXISTS `orders`;

CREATE TABLE `orders` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `customer` varchar(25) NOT NULL DEFAULT '',
  `address1` varchar(100) DEFAULT NULL,
  `city` varchar(25) NOT NULL DEFAULT '',
  `postcode` varchar(25) NOT NULL DEFAULT '',
  `country` varchar(25) NOT NULL DEFAULT '',
  `amount` varchar(11) NOT NULL DEFAULT '',
  `status` varchar(10) NOT NULL DEFAULT '',
  `deleted` varchar(10) NOT NULL DEFAULT '',
  `date` datetime NOT NULL,
  `last_modified` datetime DEFAULT NULL,
  `customers_id` int(11) NOT NULL,
  PRIMARY KEY (id),
  CONSTRAINT orders_customers_id_fk FOREIGN KEY (customers_id) 
     REFERENCES customers (id)  ON DELETE SET NULL);
  
LOCK TABLES `orders` WRITE;
/*!40000 ALTER TABLE `orders` DISABLE KEYS */;


/*!40000 ALTER TABLE `orders` ENABLE KEYS */;
UNLOCK TABLES;


# Dump of table customers
# ------------------------------------------------------------

DROP TABLE IF EXISTS `customers`;

CREATE TABLE `customers` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `role` varchar(25) NOT NULL DEFAULT '',
  `name` varchar(50) NOT NULL DEFAULT '',
  `surname` varchar(50) NOT NULL DEFAULT '',
  `email` varchar(50) NOT NULL DEFAULT '',
  `password` char(255) NOT NULL DEFAULT '',
  `created_at` datetime NOT NULL ON UPDATE CURRENT_TIMESTAMP,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

我try 了外键代码的语法，但仍然有相同的错误，我不知道为什么？

我得到了这个错误:

#1005 - Can't create table `management`.`orders` (errno: 150 "Foreign key constraint is incorrectly formed")