Menemukan respon error (HTTP/1.1 500 Internal Server Error) CodeIgniter

Saya menemui kendala pada saat download file berformat PDF dari database, ini controllernya

	// Download File
	public function download($registerd_id) {
		if(!empty($registerd_id)){
			// load download helper
			$this->load->helper('download');

			// get file info from database
			$fileInfo = $this->mDaftar_onlines->listDaftar_onlinesPub(array('registerd_id'=>$registerd_id));

			//file path
			$file = '/assets/upload/file/'.$fileInfo['file.pdf'];

			//download file
			force_download($file, NULL);
		}
	}

dan ini modelnya

		// Listing Daftar Online Publish
        public function listDaftar_onlinesPub() {
            $this->db->select('*');
            $this->db->from('daftar_onlines');
            $this->db->join('admins','admins.admin_id = daftar_onlines.user_id','LEFT');
            $this->db->where(array('status' => 'publish'));
            $this->db->order_by('registerd_id','ASC');
            $query = $this->db->get();
            return $query->result_array();
        }

kira kira ada yg bisa bantu untuk menemukan solusinya? urgent :(