Codeigniter表单验证不设置规则

我已经卡住了一段时间试图设置我的验证规则，在我的控制器上我有这个：

public function login () {
        $this->load->library('form_validation');
        $this->form_validation->set_rules(array(
            'field'=>'username',
            'label'=>'Username',
            'rules'=>'required'
            ));
        if (!$this->form_validation->run()) {
            $this->json(array(
                'status'=>'error',
                'errors'=>$this->form_validation->error_array(),
                'test'=>$this->form_validation->get_options()
                ));
            return;
        }

        $user = $this->MUser->findByAttributes(array('username'=>$this->input->post('username')));
        if(!$user) {
            $this->json(array(
                'status'=>'error',
                'errors'=>array('username'=>'Username is not yet registered.')
                ));
            return;
        } 

        $encpass = sha1($user->salt.$this->input->post('password'));

        if ($encpass != $user->password) {
            $this->json(array(
                'status'=>'error',
                'errors'=>array('password'=>'Invalid username/password, please try again.')
                ));
            return; 
        }

        $this->json(array(
            'status'=>'success',
            'errors'=>''
            ));
    }

我还有一个扩展的基本控制器类和一个扩展的表单验证类，如下所示：

class AM_Controller extends CI_Controller {
    public function __construct(){
        parent::__construct();
    }

    public function render_template($view, $data = array(), $internal = false) {
        $view_contents = $this->load->view($view, $data, true);

        $header = $internal?$this->load->view('internal/header',$data,true):$this->load->view('external/header',$data,true);
        $footer = $internal?$this->load->view('internal/footer',$data,true):$this->load->view('external/footer',$data,true);

        $this->output->set_status_header(200,'ok');

        $this->load->view('general/template',array_merge(array(
            'header'=>$header,
            'footer'=>$footer,
            'content'=>$view_contents
            ),$data));
    }

    public function json($data) {
        $this->output->set_status_header(200,'ok')
                    ->set_content_type('application/json')
                    ->set_output(json_encode($data));
    }
}

class AM_Form_validation extends CI_Form_validation {
    public function __construct($options = array()) {
        parent::__construct($options);
    }   

    public function correct_captcha ($str) {
        $this->set_message('correct_captcha','Invalid %s answer.');
        return $this->CI->session->userdata('captcha_answer') == $str;
    }

    public function get_options () {
        return $this->_field_data;
    }
}

但问题是每次我尝试测试表单验证时，无论我是否输入了某个值，它总是返回false . 知道是什么导致了这个bug吗？

顺便说一句，我也尝试过只为我的控制器类使用CI_Controller，但仍然没有运气