form_error显示“xss_clean”

I work with CodeIgniter. I need to make multiple array in my form and sort the form by the name.

My html :

<input type="text" value="<?= set_value('joueur[0][nom]') ?>" name="joueur[0][nom]">

My php :

foreach ($_POST as $form => $listPersons)
{

    foreach($listPersons as $numPerson => $listFields)
    {

        foreach ($listFields as $field => $value)
        {

             $fieldName = "{$form}[{$numPerson}][{$field}]";
             /* in switch case ...*/
                default:
                    $this   ->form_validation
                            ->set_rules($fieldName, 'Tous les champs sont obligatoires', 'trim|required|xss_clean');
                break;
        }
    }
}


        }

    }

}

And the var_dump :

string(18) "
(xss_clean)
"

My input value is "hello" or other correct string. I don't understand why the form error return (xss_clean) :-/

Thank you for your help !

It's work now. The xss_clean need to load the security Helper, and I had forgotten. You can load the security helper in the config/autoload.php or the controller.