php://output results in empty file

I have a WordPress theme template file in use by a page. The template queries the db for an array and then attempts to output the result to a csv file. No output to the browser is expected. Here is code:

/***
 * Output an array to a CSV file
 */
function download_csv_results($results, $name = NULL)
{
if( ! $name)
{
    $name = md5(uniqid() . microtime(TRUE) . mt_rand()). '.csv';
}

header('Content-Type: text/csv');
header('Content-Disposition: attachment; filename='. $name);
header('Pragma: no-cache');
header("Expires: 0");

$outstream = fopen("php://output", "w");

foreach($results as $result)
{
    fputcsv($outstream, $result);
}
fclose($outstream);
}

The file is written to the user’s downloads directory as expected, but it is empty. I’ve debugged to verify there is a result of 117 elements. The loop above is executing. It’s as though the output buffer is not being flushed or is being cleared by WordPress.