Input Problem when use Checkbox form

[eluser]dimasedu[/eluser]
I having problem when i use checkbox form to insert data into database,

This the Error code :

A PHP Error was encountered
Severity: Notice
Message: Undefined offset: 3
Filename: controllers/kelas.php
Line Number: 156
A PHP Error was encountered
Severity: Warning
Message: Cannot modify header information - headers already sent by (output started at C:\AppServ\www\simenko\system\libraries\Exceptions.php:166)
Filename: helpers/url_helper.php
Line Number: 541

and this is The Controller code:

function tambah(){
$nis = $this->input->post('cid');//this is checkbox form
$nama = $this->input->post('nama');
$kelas = $this->input->post('kelas');//this is array {<input type="hidden" name="kelas[]" value="10A" >} i put this into form

//the input code
for($i=0,$i<=count($nis);$i++){
$kode = $nis[$i];
$kls = $kelas[$i];

$this->siswa_model->inputpindahkelas($data);
}
redirect('siswa/formpindahkelas');
}

but the error appears when my program run on windows, if I use linux to run normally, I use BLANKON linux

Is there that can help me solve this problem?

[eluser]maria clara[/eluser]
have you loaded url_helper and form_helper??

Code:

$this->load->helper('url','form');

[eluser]dimasedu[/eluser]
I've put $ this-> load-> helper ( 'url', 'form'); the autoload file.
the problem is why the problem occurs only when the program runs on Windows, and running smoothly while in Linux