Uncaught TypeError: Cannot read property 'top' of undefined как исправить?

    var menu_selector = ".sticky-tabs"; 
    function onScroll() { 
        var scroll_top = $(document).scrollTop(); 
        $(menu_selector + " a").each(function () { 
            var hash = $(this).attr("href"); 
            var target = $(hash); 
            if (target.position().top <= scroll_top && target.position().top + target.outerHeight() > scroll_top) { 
                $(menu_selector + " a.active").removeClass("active"); 
                $(this).addClass("active"); 
            } else { 
                $(this).removeClass("active"); 
            } 
        }); 
    } 
    $(document).ready(function () { 
        $(document).on("scroll", onScroll); 
        $("a[href^=#]").click(function (e) { 
            e.preventDefault(); 
            $(document).off("scroll"); 
            $(menu_selector + " a.active").removeClass("active"); 
            $(this).addClass("active"); 
            var hash = $(this).attr("href"); 
            var target = $(hash); 
            $("html, body").animate({ 
                scrollTop: target.offset().top 
            }, 500, function () { 
                window.location.hash = hash; 
                $(document).on("scroll", onScroll); 
            }); 
        }); 
    });