Codeigniter upload ภาพแล้วมัน error แบบนี้ You did not select a file to upload.

<form name="post_form" enctype="multipart/form-data" action='<?php echo site_url() . '/post/admin_update'; ?>' method="post">
       
    <b>Title : </b><input type="text" name="title"  maxlength="30" value="" /><br/><br/>
    <b>Link : </b><input type="text" name="link" value="" /><br/><br/>
    <b>Image : </b><input type="file" name="image" value="" /><br/><br/>
    <input type="submit" value=" OK " />
</form>

        function admin_update()
        {

              $this->do_upload_image();

        }  
        
        
        function do_upload_image()
	{
		$config['upload_path'] = './uploads/';
		$config['allowed_types'] = 'gif|jpg|png';

               // $config['file_name'] = $userfile;
//		$config['max_size']	= '100';
//		$config['max_width']  = '1024';
//		$config['max_height']  = '768';

		$this->load->library('upload', $config);

		if ( ! $this->upload->do_upload())
		{
			$error = array('error' => $this->upload->display_errors());

			// $this->load->view('form_post_view', $error);
                        echo $this->upload->display_errors();
		}
		else
		{
			$data = array('upload_data' => $this->upload->data());
			$this->load->view('admin_view', $data);
		}
	}