Simple WordPress endpoint route doesn’t work

Exact namespace prefix and non-empty endpoint name helped me:

add_action('rest_api_init', function () {
    register_rest_route('vku/notifications', 'test', [
        'methods' => 'GET',
        'callback' => function (WP_REST_Request $request) {
            error_log('Route "notifications" works');
            return new WP_REST_Response(true, 200);
        },
    ]);
});