Updatepanel gives web request manager error

Question

I have an updatepanel that inside it I have a dropdownlist control with postback enable when the user click on that dropdownlist AjaxEditor's value must change but error happend it says "Error: Sys.WebForms.PageRequestManagerServerErrorException: Input string was not in a correct format"

                            <asp:DropDownList ID="DDLStand" runat="server" Width="200px" AutoPostBack="True"
                            DataSourceID="LinqDataSource_Stand" DataTextField="Code" DataValueField="Code"
                            OnSelectedIndexChanged="DDLStand_SelectedIndexChanged">
                        </asp:DropDownList>
                        <asp:LinqDataSource ID="LinqDataSource_Stand" runat="server" ContextTypeName="DbDataContext"
                            EntityTypeName="" Select="new (Code, ShortDescription)" TableName="Stands" Where="eHallID == @eHallID">
                            <WhereParameters>
                                <asp:ControlParameter ControlID="DDLHall" Name="eHallID" PropertyName="SelectedValue"
                                    Type="Int64" />
                            </WhereParameters>
                        </asp:LinqDataSource>
                        [read only]<custom:SimpleEditor ID="StandDescription" runat="server" 
                            BorderColor="#999999" BorderStyle="Solid"
                            BorderWidth="1px" Height="150px" Width="220px" />