Fatal error on PHP 8.5

Upon upgrading sites to PHP 8.5, we started getting this error:

[08-Jul-2026 12:54:38 UTC] PHP Fatal error: Uncaught TypeError: Unsupported operand types: int – string in /home/dcppq/public_html/wp-content/plugins/download-manager/src/__/FileSystem.php:176
Stack trace:
#0 /home/dcppq/public_html/wp-content/plugins/download-manager/src/wpdm-functions.php(57): WPDM\__\FileSystem::downloadFile(‘/home/dcppq/pub…’, ‘2020-11-04_area…’, 4194304, 1, Array)
#1 /home/dcppq/public_html/wp-content/plugins/download-manager/src/wpdm-start-download.php(107): wpdm_download_file(‘/home/dcppq/pub…’, ‘2020-11-04_area…’, 4096, 1, Array)
#2 /home/dcppq/public_html/wp-content/plugins/download-manager/src/__/Apply.php(316): include(‘/home/dcppq/pub…’)
#3 /home/dcppq/public_html/wp-includes/class-wp-hook.php(341): WPDM\__\Apply->triggerDownload(”)
#4 /home/dcppq/public_html/wp-includes/class-wp-hook.php(365): WP_Hook->apply_filters(NULL, Array)
#5 /home/dcppq/public_html/wp-includes/plugin.php(522): WP_Hook->do_action(Array)
#6 /home/dcppq/public_html/wp-settings.php(771): do_action(‘init’)
#7 /home/dcppq/public_html/wp-config.php(112): require_once(‘/home/dcppq/pub…’)
#8 /home/dcppq/public_html/wp-load.php(50): require_once(‘/home/dcppq/pub…’)
#9 /home/dcppq/public_html/wp-blog-header.php(13): require_once(‘/home/dcppq/pub…’)
#10 /home/dcppq/public_html/index.php(17): require(‘/home/dcppq/pub…’)
#11 {main}
thrown in /home/dcppq/public_html/wp-content/plugins/download-manager/src/__/FileSystem.php on line 176

It looks like a fix could be something along these lines, on line 158 of FileSystem.php:

if ( substr( $http_range, 0, 1 ) === ‘-‘ ) {
$start_range = $file_size – (int) substr( $http_range, 1 );
if ( $start_range < 0 ) {
$start_range = 0;
}

I’ve checked both the wordpress.org forum and this one, and haven’t seen anyone else reporting this. We have many sites running Download Manager, and this fix worked for one. Perhaps there’s something else we can do. Please let me know. Thank you!