Codeigniter和Ion Auth：无法使特定的寄存器字段唯一

Trying to make email field unique throws the following error: 'The Email field must contain a unique value.' But the parameter not exist and it's unique so, what's worng?

The code

$this->form_validation->set_rules('email','Email','trim|is_unique[users.email]|valid_email|required');

Load database

       function __construct() {
        parent::__construct();
        $this->load->database();
     }

looking for answer at same problem I found the solution on DB:

** there was a previous user registered with same email **