Stuck di form_validation cover di function rules saat upload gambar di codeigniter.

Isi kode controller buku.php

<?php

class Buku extends CI_Controller{

	public function index()
	{
		$data['buku']	= $this->buku_model->tampil_data()->result();
		$this->load->view('templates_administrator/header');
		$this->load->view('templates_administrator/sidebar');
		$this->load->view('administrator/buku',$data);
		$this->load->view('templates_administrator/footer');
	}

	public function input()
	{
		$data['buku'] = $this->buku_model->tampil_data('buku')->result();
		$data = array(
			'id'			=> set_value('id'),
			'judul'			=> set_value('judul'),
			'pengarang'		=> set_value('pengarang'),
			'kategori_id'	=> set_value('kategori_id'),
			'penerbit'		=> set_value('penerbit'),
			'cover'			=> set_value('cover'),
			'tahun_terbit'	=> set_value('tahun_terbit'),
			'volum'			=> set_value('volum'),
			'halaman'		=> set_value('halaman'),
			'lokasi'		=> set_value('lokasi'),
			'stok'			=> set_value('stok'),
		);
		$data['penerbit']	= $this->penerbit_model->tampil_data('penerbit')->result();
		$data['kategori']	= $this->kategori_model->tampil_data('kategori')->result();
		$this->load->view('templates_administrator/header');
		$this->load->view('templates_administrator/sidebar');
		$this->load->view('administrator/buku_form',$data);
		$this->load->view('templates_administrator/footer');
	}

	public function input_aksi()
	{
		$this->_rules();

		if($this->form_validation->run() == FALSE) {
			$this->input();
		}else{
			$judul 			= $this->input->post('judul');
			$pengarang 		= $this->input->post('pengarang');
			$kategori_id 	= $this->input->post('kategori_id');
			$penerbit 		= $this->input->post('penerbit');
			$tahun_terbit 	= $this->input->post('tahun_terbit');
			$volum 			= $this->input->post('volum');
			$halaman 		= $this->input->post('halaman');
			$lokasi 		= $this->input->post('lokasi');
			$stok 			= $this->input->post('stok');
			$cover 			= $this->input->post('cover');
			if ($cover=''){}else{
				$config['upload_path'] = './assets/uploads';
				$config['allowed_types'] = 'jpg|png|giff';

				$this->load->library('upload',$config);
				if(!$this->upload->do_upload('cover')){
					echo"Gagal Upload"; die();
				}else{
					$cover=$this->upload->data('file_name');
				}
			}

			$data = array(
				'judul' 		=> $judul,
				'pengarang' 	=> $pengarang,
				'kategori_id'	=> $kategori_id,
				'penerbit' 		=> $penerbit,
				'cover' 		=> $cover,
				'tahun_terbit' 	=> $tahun_terbit,
				'volum'	 		=> $volum,
				'halaman' 		=> $halaman,
				'lokasi' 		=> $lokasi,
				'stok' 			=> $stok,

			);
			$this->buku_model->input_data($data,'buku');
			$this->session->set_flashdata('pesan','<div class="alert alert-danger alert-dismissible fade show" role="alert">
												  Data Buku Berhasil Ditambahkan!
												  <button type="button" class="close" data-dismiss="alert" aria-label="Close">
												    <span aria-hidden="true">×</span>
												  </button>
												</div>');
			redirect('administrator/buku');
		}
	}

public function _rules()
{
	$this->form_validation->set_rules('judul','judul','required',['required'=> 'Judul wajib diisi!']);
	$this->form_validation->set_rules('pengarang','pengarang','required',['required'=> 'Pengarang wajib diisi!']);
	$this->form_validation->set_rules('kategori_id','kategori_id','required',['required'=> 'Kategori wajib diisi!']);
	$this->form_validation->set_rules('penerbit','penerbit','required',['required'=> 'Penerbit wajib diisi!']);
	$this->form_validation->set_rules('cover','cover','required',['required'=> 'Cover wajib diupload!']);
	$this->form_validation->set_rules('tahun_terbit','tahun_terbit','required',['required'=> 'Tahun Terbit wajib diisi!']);
	$this->form_validation->set_rules('volum','volum','required',['required'=> 'Volum wajib diisi!']);
	$this->form_validation->set_rules('halaman','halaman','required',['required'=> 'Jumlah Halaman wajib diisi!']);
	$this->form_validation->set_rules('lokasi','lokasi','required',['required'=> 'Lokasi Penyimpanan wajib diisi!']);
	$this->form_validation->set_rules('stok','stok','required',['required'=> 'Jumlah stok wajib diisi!']);
}

public function update($id)
{
	$where = array('id'=>$id);
	$data['buku'] = $this->buku_model->edit_data($where,'buku')->result();
	$data['buku'] = $this->db->query("select * from buku buk, kategori kat where buk.kategori_id=kat.id_kategori and buk.id='$id'")->result();
	$data['buku'] = $this->buku_model->edit_data($where,'buku')->result();
	$data['penerbit']	= $this->penerbit_model->tampil_data('penerbit')->result();
	$data['kategori']	= $this->kategori_model->tampil_data('kategori')->result();
	$this->load->view('templates_administrator/header');
		$this->load->view('templates_administrator/sidebar');
		$this->load->view('administrator/buku_update',$data);
		$this->load->view('templates_administrator/footer');
}

public function update_aksi()
{
	$id 			= $this->input->post('id');
	$judul 			= $this->input->post('judul');
	$pengarang 		= $this->input->post('pengarang');
	$kategori_id 	= $this->input->post('kategori_id');
	$penerbit 		= $this->input->post('penerbit');
	$cover 			= $this->input->post('cover');
	$tahun_terbit 	= $this->input->post('tahun_terbit');
	$volum 			= $this->input->post('volum');
	$halaman 		= $this->input->post('halaman');
	$lokasi 		= $this->input->post('lokasi');
	$stok 			= $this->input->post('stok');
	$data = array(
		'judul' 		=> $judul,
		'pengarang' 	=>$pengarang,
		'kategori_id' 	=> $kategori_id,
		'penerbit' 		=> $penerbit,
		'cover' 		=> $cover,
		'judul' 		=> $judul,
		'tahun_terbit' 	=> $tahun_terbit,
		'volum' 		=> $volum,
		'halaman' 		=> $halaman,
		'lokasi' 		=> $lokasi,
		'stok' 			=> $stok,
	);
	$where = array(
		'id' => $id
	);
	$this->buku_model->update_data($where,$data,'buku');

	$this->session->set_flashdata('pesan','<div class="alert alert-success alert-dismissible fade show" role="alert">
												  Data Buku Berhasil Diupdate!
												  <button type="button" class="close" data-dismiss="alert" aria-label="Close">
												    <span aria-hidden="true">×</span>
												  </button>
												</div>');
	redirect('administrator/buku');
}

public function delete($id)
{
	$where = array('id' => $id);
	$this->buku_model->hapus_data($where,'buku');
	$this->session->set_flashdata('pesan','<div class="alert alert-danger alert-dismissible fade show" role="alert">
												  Data Buku Berhasil Dihapus!
												  <button type="button" class="close" data-dismiss="alert" aria-label="Close">
												    <span aria-hidden="true">×</span>
												  </button>
												</div>');
	redirect('administrator/buku');
}
}

Isi kode view buku.php

<div class="container-fluid">
	<div class="alert alert-success" role="alert">
  <i class="fa fa-tachometer-alt"></i> Form Input Buku
</div>

<?php echo form_open_multipart('administrator/buku/input_aksi')?>
		<div class="form-group">
			<label>Judul</label>
			<input type="text" name="judul" placeholder="Masukkan Judul" class="form-control">
			<?php echo form_error('judul','<div class="text-danger small" ml-3>')?>
		</div>
		<div class="form-group">
			<label>Pengarang</label>
			<input type="text" name="pengarang" placeholder="Masukkan Nama Pengarang" class="form-control">
			<?php echo form_error('pengarang','<div class="text-danger small" ml-3>')?>
		</div>
		<div class="form-group">
			<label>Kategori</label>
			<select name="kategori_id" class="form-control">
				<option value="">Pilih Kategori</option>
				<?php foreach($kategori as $kat) : ?>
				<option value="<?php echo $kat->id_kategori ?>"><?php echo $kat->id_kategori; ?></option>
			<?php endforeach; ?>
		</select>
		</div>
		<div class="form-group">
			<label>Penerbit</label>
 		<select name="penerbit" class="form-control">
 			<option value="">Pilih Penerbit</option>
 			<?php foreach ($penerbit as $pen) :?>
 			<option value="<?php echo $pen->nama_penerbit?>"><?php echo $pen->nama_penerbit; ?></option>
 			<?php endforeach ?>
 		</select>
		</div>
		<div class="form-group">
			<label>Tahun Terbit</label>
			<input type="text" name="tahun_terbit" placeholder="Masukkan Tahun Terbit" class="form-control">
			<?php echo form_error('tahun_terbit','<div class="text-danger small" ml-3>')?>
		</div>
		<div class="form-group">
			<label>Volum</label>
			<input type="text" name="volum" placeholder="Masukkan Volum Buku" class="form-control">
			<?php echo form_error('volum','<div class="text-danger small" ml-3>')?>
		</div>
		<div class="form-group">
			<label>Jumlah Halaman</label>
			<input type="text" name="halaman" placeholder="Masukkan Jumlah Halaman" class="form-control">
			<?php echo form_error('halaman','<div class="text-danger small" ml-3>')?>
		</div>
		<div class="form-group">
			<label>Lokasi Penyimpanan</label>
			<input type="text" name="lokasi" placeholder="Masukkan Lokasi Penyimpanan" class="form-control">
			<?php echo form_error('lokasi','<div class="text-danger small" ml-3>')?>
		</div>
		<div class="form-group">
			<label>Stok</label>
			<input type="text" name="stok" placeholder="Masukkan Jumlah Stok Buku" class="form-control">
			<?php echo form_error('stok','<div class="text-danger small" ml-3>')?>
		</div>
		<div class="form-group">
			<label>Cover</label><br>
			<?php echo "<input type='file' name='cover' size='20'>"; ?>
			<?php echo form_error('cover','<div class="text-danger small" ml-3>')?>
		</div>

		<button type="submit" class="btn btn-primary">Simpan</button>
	</form>
	<?php form_close() ?>
	</div>

Isi kode model buku_model.php

<?php

class Buku_model extends CI_Model{
	public function tampil_data()
	{
		return $this->db->get('buku');
	}

	public function input_data($data)
	{
		$this->db->insert('buku',$data);
	}

	public function edit_data($where,$table)
	{
		return $this->db->get_where($table,$where);
	}
	public function update_data($where,$data,$table)
	{
		$this->db->where($where);
		$this->db->update($table,$data);
	}
	public function hapus_data($where,$table)
	{
		$this->db->where($where);
		$this->db->delete($table);
	}
}