Codeigniter and Ion Auth: Cannot make specific register field unique

Question

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');

Answer 1

Load database

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