jquery autocomplete forcing the user to select an item from the list

Looking for some help to force user to select an item from a jquery autocomplete textbox.  What I have found doesn't seem to work for me & I am not sure why.

Here is a demo:  http://members.gcp21.org/autocomplete.aspx

    <script type="text/javascript">
        $(document).ready(function () {
            $("#dtv_Country").autocomplete({
                source: function (request, response) {
                    $.ajax({
                        type: "POST",
                        contentType: "application/json; charset=utf-8",
                        url: "autocomplete.aspx/getCountries",
                        data: "{'prefixText':'" + $("#dtv_Country").val() + "'}",
                        dataType: "json",
                        success: function (data) {
                            response(data.d);
                        },
                        error: function (result) {
                            alert("Error");
                        },
                        select: function (e, ui) {
                                $(this).next().val(ui.item.id);
                        },
                        change: function (ev, ui) {
                        if (!ui.item)
                            $(this).val("");
                    }
                  });
                }
            });

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