Codeigniter 3.0 captcha 404

我在codeigniter(3.0)中有帮助captcha问题。验证码工作良好，但在日志中是:

ERROR - 2016-09-22 08:32:25 --> 404 Page Not Found: [207.46.XXX.XXX] > /upload/captcha/1470072358.0468.jpg

，每天约有100个错误。

问题在哪里?

我的控制器:

$random_number = substr(number_format(time() * rand(), 0, '', ''), 0, 6);
        // setting up captcha config
        $vals = array(
            'word'       => $random_number,
            'img_path'   => './upload/captcha/',
            'img_url'    => base_url() . 'upload/captcha/',
            'img_width'  => 140,
            'img_height' => 32,
            'expiration' => 7200,
            'pool'       => '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ',
            'colors'     => array(
                'background' => array(255, 255, 255),
                'border'     => array(255, 255, 255),
                'text'       => array(111, 34, 45),
                'grid'       => array(255, 40, 40),
            ),
            'font_path'  => base_url() . 'system/fonts/texb.ttf',
        );
        $data            = array();
        $data['captcha'] = create_captcha($vals);
        $this->session->set_userdata('captchaWord', $data['captcha']['word']);
        $this->view_data['captcha'] = create_captcha($vals);