file format is incorrect in PHPExcel Code - phpexcel

Question

I am trying to write mySQL data into Exlcel using PHPExcel, but I am getting file format is invalid in my above PHP code using PHPExcel. Can someone please help me to find out what is wrong in code:
$query = "SELECT company_id, userid FROM uam_user";
$result = mysql_query($query) or die ("Error in Query".mysql_error());
// Instantiate a new PHPExcel object
$objPHPExcel = new PHPExcel();
// Set document properties
$objPHPExcel->getProperties()->setCreator("Security Team")
->setLastModifiedBy("System")
->setTitle("System")
->setSubject("System Security")
->setDescription("Non Production user Upload Data")
->setKeywords("User Upload")
->setCategory("Security");
// Set the active Excel worksheet to sheet 0
$objPHPExcel->setActiveSheetIndex(0);
$objPHPExcel->getActiveSheet()->SetCellValue("A1", 'company_id');
$objPHPExcel->getActiveSheet()->SetCellValue("B2", 'userid');
//start of printing column names as names of MySQL fields
$rowCount = 2;
//$num_rows = mysql_num_rows($result);
//echo $num_rows;
$i = 0;
while($row = mysql_fetch_array($result, MYSQL_ASSOC)){
if ($row['userid'] != '')
{
//echo "A$rowCount=".$row['company_id']."|B$rowCount=".$row['userid']."<br>";
$objPHPExcel->getActiveSheet()->SetCellValue("A$rowCount",$row['company_id']);
$objPHPExcel->getActiveSheet()->SetCellValue("B$rowCount",$row['userid']);
}
$rowCount++;
//echo $rowCount;
}
$objWriter = PHPExcel_IOFactory::createWriter($objPHPExcel, 'Excel2007');
header('Content-Type: application/vnd.ms-excel');
header('Content-Disposition: attachment;filename="item_list.xlsx"');
header('Cache-Control: max-age=0');
ob_end_clean();
$objWriter->save ( 'php://output' );