Why is the variable not added to query_vars?

Question

WordPress

add_filter('query_vars', 'success_query_vars');
function success_query_vars($vars)
{
    $vars[] .= 'my_success';
    return $vars;
}

add_action('parse_request', 'success_parse_request');
function success_parse_request($wp)
{
    if (array_key_exists('my_success', $wp->query_vars)) { /* не заходит сюда */ }
}

Answer 1

query_vars - what is this hook? Mine? It works, have you checked?