Error a database oncurred

ini file Controler

<?php
class Mahasiswa extends CI_Controller
{

	public function index()
	{
		$data['mahasiswa']  = $this->mahasiswa_model->tampil_data('mahasiswa')->result();

		$this->load->view('templates_administrator/header');
		$this->load->view('templates_administrator/sidebar');
		$this->load->view('administrator/mahasiswa', $data);
		$this->load->view('templates_administrator/footer');
	}
	public function detail($id)

	{
		$data['detail']  = $this->mahasiswa_model->ambil_id_mahasiswa($id);
		$this->load->view('templates_administrator/header');
		$this->load->view('templates_administrator/sidebar');
		$this->load->view('administrator/mahasiswa_detail', $data);
		$this->load->view('templates_administrator/footer');
	}
	public function tambah_mahasiswa()
	{
		$data['prodi'] = $this->mahasiswa_model->tampil_data('mahasiswa')->result();
		$this->load->view('templates_administrator/header');
		$this->load->view('templates_administrator/sidebar');
		$this->load->view('administrator/mahasiswa_form', $data);
		$this->load->view('templates_administrator/footer');
	}
	public function tambah_mahasiswa_aksi()
	{
		$this->_rules();

		if ($this->form_validation->run() == FALSE) {
			$this->tambah_mahasiswa();
		} else {
			$nim			= $this->input->post('nim');
			$nama_lengkap	= $this->input->post('nama_lengkap');
			$alamat			= $this->input->post('alamat');
			$email			= $this->input->post('email');
			$telepon		= $this->input->post('telepon');
			$tempat_lahir	= $this->input->post('tempat_lahir');
			$tanggal_lahir	= $this->input->post('tanggal_lahir');
			$jenis_kelamin	= $this->input->post('jenis_kelamin');
			$nama_prodi		= $this->input->post('nama_prodi');
			$photo			= $_FILES['photo'];
			if ($photo = '') {
			} else {
				$config['upload_path'] = './assets/uploads';
				$config['allowed_types'] = 'jpg|png|gift|tift';

				$this->load->library('upload', $config);
				if (!$this->upload->do_upload('photo')) {
					echo "Gagal Upload";
					die();
				} else {
					$photo = $this->upload->data('files_name');
				}
			}
			$data 				= array(
				'nim'			=> $nim,
				'nama_lengkap'	=> $nama_lengkap,
				'alamat'		=> $alamat,
				'email'			=> $email,
				'telepon'		=> $telepon,
				'tempat_lahir'	=> $tempat_lahir,
				'tanggal_lahir'	=> $tanggal_lahir,
				'jenis_kelamin'	=> $jenis_kelamin,
				'nama_prodi'	=> $nama_prodi,
				'photo'			=> $photo

			);
			$this->mahasiswa_model->insert_data($data, 'mahasiswa');
			$this->session->set_flashdata('pesan', '<div class="alert alert-success alert-dismissible fade show" role="alert">
                   Data Mahasiswa Berhasil ditambahkan!
                    <button type="button" class="close" data-dismiss="alert" aria-label="Close">
                      <span aria-hidden="true">×</span>
                    </button>
                  </div>');
			redirect('administrator/mahasiswa');
		}
	}
	public function _rules()
	{
$this->form_validation->set_rules('nim', 'Nim', 'required', ['required' => 'Nim wajib diisi!']);
$this->form_validation->set_rules('nama_lengkap', 'Nama Lengkap', 'required', ['required' => 'Nama Lengkap wajib diisi!']);
$this->form_validation->set_rules('alamat', 'Alamat', 'required', ['required' => 'Alamat wajib diisi!']);
$this->form_validation->set_rules('email', 'Email', 'required', ['required' => 'Email wajib diisi!']);
$this->form_validation->set_rules('telepon', 'Telepon', 'required', ['required' => 'Telepon wajib diisi!']);
$this->form_validation->set_rules('tempat_lahir', 'Tempat Lahir', 'required', ['required' => 'Tempat lahir wajib diisi!']);
$this->form_validation->set_rules('tanggal_lahir', 'Tanggal Lahir', 'required', ['required' => 'Tanggal Lahir wajib diisi!']);
$this->form_validation->set_rules('jenis_kelamin', 'Jenis Kelamin', 'required', ['required' => 'Jenis Kelamin wajib diisi!']);
$this->form_validation->set_rules('nama_prodi', 'Nama Prodi', 'required', ['required' => 'Nama Prodi wajib diisi!']);
	}
}

error number : 1048 Error Number: 1048

Column 'photo' cannot be null

INSERT INTO `mahasiswa` (`nim`, `nama_lengkap`, `alamat`, `email`, `telepon`, `tempat_lahir`, `tanggal_lahir`, `jenis_kelamin`, `nama_prodi`, `photo`) VALUES ('80401732', 'Ikhsan Madani', 'Bogor Utara Selatan Barat', 'mitradarmaz@gmail.com', '08556789765', 'Nias', '1988-11-22', 'Laki-laki', 'Sistem Informasi', NULL)

Filename: C:/xampp/htdocs/sisfo_akademik/system/database/DB_driver.php

Line Number: 691