Object doesn't support this property or method - jquery - IE8

Question

i'm beginner, so I have a question with my issues.

I used jquery 1.11.1. On IE8, it's happen when page loaded, error on line 2 ( $(_label) is undefined )

But i checked $(_label).toString() is valid.

Here my code:

function AutocompleteDropDownMultiValue(_label, _hidden, _mySource) {
    $(_label).autocomplete({
            source: _mySource,
            maxHeight: 500,
            width: 300,
            select: function (event, ui) {
                var terms = split($(this).text());
                var index = SearchItem(terms, ui.item.value);
                if (index == -1) {
                    if (terms[0] == "Click to select")
                        terms.pop();
                    terms.push(ui.item.value);
                    $(this).html("<b>" + terms.join(", ") + "</b>");
                    $(_hidden).val(terms.join("|"));
                } else {
                    terms.splice(index, 1);
                    if (terms.length == 0) {
                        terms.push("Click to select");
                        $(this).html(terms);
                        $(_hidden).val("");
                    } else {
                        $(this).html("<b>" + terms.join(",") + "</b>");
                        $(_hidden).val(terms.join("|"));
                    }
                }
                return false;
            },
            open: function () {
                $('.ui-autocomplete').css('width', '280px');
                $('.ui-autocomplete').css('height', 'auto');
                $('.ui-autocomplete').css('max-height', '800px');
                $('.ui-autocomplete').css('overflow-y', 'auto');
            }
        })
        .data("ui-autocomplete")._renderItem = function (ul, item) {
            var terms = split($(_label).text());
            var index = SearchItem(terms, item.value);
            if (index == -1) {
                return $("<li>")
                    .append("<a>" + item.label + "</a>")
                    .appendTo(ul);
            } else {
                return $("<li>")
                    .data("ui-autocomplete-item", item)
                    .append("<a style='color:#ff5500;'>" + item.label + "</a>")
                    .appendTo(ul);
            }
        };
}

function ShowDropDownMultiValue(_label) {
    $(_label).autocomplete("option", "minLength", 0);
    if ($(_label).autocomplete("widget").is(':visible')) {
        $(_label).autocomplete('close');
        return;
    } else {
        $(_label).autocomplete("search", "");
        $(_label).autocomplete("widget").show();
        return;
    }
}

Thanks so much.