Management page broken with parameters when not using trailing slashes

My permalink structure is /%year%/%monthnum%/%postname%.html (to line up with a very old version of the site), which means that all URLs (posts, pages, categories, etc) do not have trailing slashes.

If I try to subscribe to a post without commenting, I get taken to a page starting with:

/comment-subscriptions?srp=xxx....

This gives a 404.

Lines 150-151 of wp_subscribe_reloaded.php:

// don't show management page if a "child page"
if ( empty( $request_uri_arr[1] ) || $request_uri_arr[1] == '/' || strpos( $request_uri_arr[1], '/?' ) === 0 ) {

This if condition returns false, because $request_uri_arr[1] is ?srp=xxx...

The last if condition seems to assume a trailing slash. You probably need another condition for handling URLs starting with a question mark directly.