Cannot modify header information - headers already sent by (output sta

Acho que tá tudo certinho, mas.... nao roda...

o que há de errado?

<?php
	ob_start();
	header("Content-type: application/octet-stream");
	header("Content-Disposition: attachment; filename=excel_".date('d').date('m').date('y').date('H').date('i').date('s').".xls");
	header("Pragma: no-cache");
	header("Expires: 0");
?>

ps.: nao roda no servidor, no local funciona..