tidak bisa update data di codeigniter

jadi gini saya punya form update data,tapi kenapa data yang lain bisa update kecuali nik. ketika merubah nik tidak mau update.

 public function updatedosen() {

        $id      = $this->input->post('nik_dosen');
        $nama	   = $this->input->post('nama_dosen');
        $jk  = $this->input->post('jk');
        $agama  = $this->input->post('agama');
        $tempat  = $this->input->post('tempat');
        $tanggal  = $this->input->post('tanggal');
        $alamat  = $this->input->post('alamat');
        $email  = $this->input->post('email');
        $hp  = $this->input->post('hp');
        $title  = $this->input->post('title');
        $status  = $this->input->post('status');
        $prodi  = $this->input->post('prodi');
        $password  = $this->input->post('confirm_password');

        if (empty($password)) {
            $data = array(

	            'nama_dosen' 	=> $nama,
	            'jenis_kelamin' => $jk,
	            'agama'			=> $agama,
	            'tempat_lahir'	=>$tempat,
	            'tgl_lahir'  	=> date('Y-m-d', strtotime($tanggal)),
	            'alamat' 		=> $alamat,
	            'email'  		=> $email,
	            'hp'  			=> $hp,
	            'status'  		=> $status,
	            'title_dosen'  	=> $title,

	            'kd_prodi'		=> $prodi,
            );
        } else {
            $data = array(

	            'nama_dosen' 	=> $nama,
	            'jenis_kelamin' => $jk,
	            'agama'			=> $agama,
	            'tempat_lahir'	=>$tempat,
	            'tgl_lahir'  	=> date('Y-m-d', strtotime($tanggal)),
	            'alamat' 		=> $alamat,
	            'email'  		=> $email,
	            'hp'  			=> $hp,
	            'status'  		=> $status,
	            'title_dosen'  	=> $title,
	            'pass_dosen'  	=> md5($password),
	            'kd_prodi'		=> $prodi,
            );

        }

        $this->dosen_model->updatedosen($id, $data);

        redirect('dosen');

    }

my model

 function updatedosen($id,$data)
    {
        $this->db->where('nik_dosen',$id);
        $update=$this->db->update('dosen',$data);
        return $updatedosen;
    }

my view

  <form class="cmxform form-horizontal tasi-form" id="DosenFormUpdate" method="post" action="<?php echo site_url('dosen/updatedosen'); ?>">

  <div class="form-group ">
  <label class="control-label col-lg-2">NIK
  <span class="required"> * </span></label>
  <div class="col-lg-6">
    <input class=" form-control" id="nik" name="nik_dosen" type="number" value="<?php echo $edit[0]->nik_dosen; ?>" onpaste="return false" />
   </div>
   </div>
<div class="form-group ">
 <label  class="control-label col-lg-2">Nama Lengkap
   <span class="required"> * </span></label>
    <div class="col-lg-6">
  <input class=" form-control" id="nama" name="nama_dosen" type="text" value="<?php echo $edit[0]->nama_dosen; ?>" />
 </div>
 </div>

merubah nama saja berhasil tapi ketika merubah nik tidak bisa.