media_sideload_image doesn't attach image to post

It appears that media_sideload_image only half works. The image is downloaded to my server and stored in the /wp-content/uploads folder as I expect, but it’s not associated with a post and the site just gives me a blank page instead of moving forward.

Here is a simplified example to test:

if (is_page('Upp')){
    require_once(ABSPATH . 'wp-admin/includes/file.php');
    require_once(ABSPATH . 'wp-admin/includes/media.php');
    set_time_limit(300);
    $upload = media_sideload_image("https://29.media.tumblr.com/tumblr_lmp1tbiYwh1qzlfumo1_500.jpg", "202", "my description");
//media_sideload_image(url,postID,description)
    die ( $upload );
}

What am I doing wrong here?

Thanks,
Dennis