Undefined index HTTP_IF_MODIFIED_SINCE

Getting the following notice:

Notice: Undefined index: HTTP_IF_MODIFIED_SINCE in F:\sitesroot\wp-content\plugins\wp-super-cache\wp-cache-phase1.php on line 237

Plugin version: 1.4
Running on IIS on Microsoft Azure Cloud.

From file: wp-cache-phase1.php, line 237:

// don't try to match modified dates if using dynamic code.
if ( $wp_cache_mfunc_enabled == 0 && $wp_supercache_304 ) {
if ( function_exists( 'apache_request_headers' ) ) {
$request = apache_request_headers();
$remote_mod_time = $request[ 'If-Modified-Since' ];
} else {
$remote_mod_time = $_SERVER[ 'HTTP_IF_MODIFIED_SINCE' ];
}
$local_mod_time = gmdate("D, d M Y H:i:s",filemtime( $file )).' GMT';
if ( $remote_mod_time == $local_mod_time ) {
header("HTTP/1.0 304 Not Modified");
exit();
}
header( 'Last-Modified: ' . $local_mod_time );
}

https://wordpress.org/plugins/wp-super-cache/