绑定下拉与webservice和ajax获取错误如何解决此问题？

Question

大家好，我在mvc和jquery

中使用webmethod绑定下拉列表时遇到错误

无法加载资源：服务器响应状态为500（内部服务器错误）

我的代码就像这样

web服务

    [WebMethod]
    public static List<ListItem> BindDESG()
    {
        AdoConClass getado = new AdoConClass();
        string query = "select Pk_DeptId,DepartmentName from emp_Master_Department where IsActive=1";
        DataTable dt=getado.retrievedata(query);
        List<ListItem> Desg = new List<ListItem>();

        if (dt.Rows.Count>0)
        {
            Desg.Add(new ListItem
            {
                Value = dt.Rows[0]["Pk_DeptId"].ToString(),
                Text = dt.Rows[0]["DepartmentName"].ToString()
            });
        }
        return Desg;
    }

Jquery的

$(document).ready(function () {
$.ajax({
    type: "POST",
    url: "../../Services/NewEMP/BindDesgDDL.asmx/BindDESG",
    data: "{}",
    contentType: 'application/json; charset=utf-8',
    success: function (r) {
        var ddlCustomers = $("[id*=sDesg]");
        ddlCustomers.empty().append('<option selected="selected" value="0">Please select</option>');
        $.each(r.d, function () {
            ddlCustomers.append($("<option></option>").val(this['Value']).html(this['Text']));
        });
    },
    error: function (msg) { alert(msg); }
});

Html标记

<select class="form-control" id="sDesg"></select>