Adding custom query vars breaks my site

Hi,

I’m building a new site on WordPress 4.4 and it requires me to use custom query vars for some sections. I’ve been using this feature for a couple of years now and things have been pretty smooth so far. However, when I added my usual code for registering the custom query vars on this site, it gives out a Server Error (#500).

Here’s the code I’m using…

//Add custom query var filter
function parameter_queryvars( $qvars ) {
    $qvars[] = 'req_id';
    $qvars[] = 'service_name';
    return $qvars;
}
add_filter('query_vars', 'parameter_queryvars' )

I’m at a loss of thoughts as to why this is happening since the same code is running fine on other websites on my localhost as well as live server (not version 4.4 though). Has there been some change in 4.4 that requires me to do things differently?

Please help!

Thanks ??