Codeigniter迁移自动和密钥问题

我正试图在Codeigniter中创建一个迁移文件，但当我运行它时，它会抛出一个错误，无论我做什么，我都无法让它运行。

class Migration_Add_users extends CI_Migration {     
public function up()
{
        $this->dbforge->add_field(array(
            'USERS_id' => array(
                'type' => 'INT',
                'constraint' => 5,
                'auto_increment' => TRUE,
            ),
            'USERS_firstname' => array(
                'type' => 'VARCHAR',
                'constraint' => '100',
            ),
            'USERS_surname' => array(
                'type' => 'VARCHAR',
                'constraint' => '100',
            ),
            'USERS_email' => array(
                'type' => 'VARCHAR',
                'constraint' => '150',
                ),
            'USERS_password' => array(
                'type' => 'VARCHAR',
                'constraint' => '150',
                ),
            'USERS_password' => array(
                'type' => 'VARCHAR',
                'constraint' => '150',
                ),
            'USERS_created' => array(
                'type' => 'DATETIME',
                ),
        ));
        $this->dbforge->create_table('users');
        $this->dbforge->add_key('USERS_id', TRUE);
    }
    public function down()
    {
        $this->dbforge->drop_table('users');
    }
}

这是错误

Error Number: 1075
Incorrect table definition; there can be only one auto column and it must be defined as a key

表格

CREATE TABLE `users` ( 
`USERS_id` INT(5) NOT NULL AUTO_INCREMENT, 
`USERS_firstname` VARCHAR(100) NOT NULL, 
`USERS_surname` VARCHAR(100) NOT NULL, 
`USERS_email` VARCHAR(150) NOT NULL, 
`USERS_password` VARCHAR(150) NOT NULL, 
`USERS_created` DATETIME NOT NULL ) DEFAULT 
CHARACTER SET utf8 COLLATE utf8_general_ci;