PHPExcel致命错误GarbageCollect（）

Question

Good day, i'm trying to export data from my table to excel with PHPExcel 1.8. Here is my script so far :

$this->load->library('excels');
                $objPHPExcel = new PHPExcel();
                $objPHPExcel->getProperties()->setTitle("export")->setDescription("none");
                $objPHPExcel->setActiveSheetIndex(0);
                $query = $this->db->query("SELECT * FROM RESIGN");
                $fields = $query->list_fields();
                $col = 0;
                foreach ($fields as $field)
                {
                    $objPHPExcel->getActiveSheet()->setCellValueByColumnAndRow($col, 1, $field);
                    $col++;
                }
                $row = 2;
                foreach($query->result() as $data)
                {
                    $col = 0;
                    foreach ($fields as $field)
                    {
                        $objPHPExcel->getActiveSheet()->setCellValueByColumnAndRow($col, $row, $data->$field);
                        $col++;
                    }
                    $row++;
                }
                $objPHPExcel->setActiveSheetIndex(0);
                $objWriter = PHPExcel_IOFactory::createWriter($this->excel, 'Excel5');
                header('Content-Type: application/vnd.ms-excel');
                header('Content-Disposition: attachment;filename="Products_'.date('dMy').'.xls"');
                header('Cache-Control: max-age=0');
                $objWriter->save('php://output');

when i run that function. Excel file will be downloaded , inside the excel file i receive this error.

<b>Fatal error</b>: Call to a member function garbageCollect() on null in <b>D:\xampp\htdocs\payroll\application\third_party\PHPExcel\Writer\Excel5.php</b> on line <b>115</b><br />