WordPress jQuery version issues

Can anyone see any reason why this simple script doesn’t work with the WordPress included jquery (v1.11 -1.12)?

<script type="text/javascript">

$(document).ready(function () {

    $(window).scroll(function () {
        if ($(this).scrollTop() > 500) {
            $('.scrollup').fadeIn();
        } else {
            $('.scrollup').fadeOut();
        }
    });

    $('.scrollup').click(function () {
        $("html, body").animate({
            scrollTop: 0
        }, 900);
        return false;
    });

});
</script>

It’s a simple script for a back-to-top button. If I manually load jquery 2.x from Google or elsewhere, it works just fine.