How to validate ajax cascading dropdown lists

I need to validate ajax cascading dropdown lists. See code below. It does not work. Could anyone help on it. I use VS 2008. Thanks

<asp:DropDownListID="ddlTest" runat="server"Width="131px"></asp:DropDownList>

<asp:RequiredFieldValidatorID="rfvddlTest"runat="server"

ControlToValidate="ddlTest"ErrorMessage="This field is Required"InitialValue="-1"ValidationGroup="Submit">*</asp:RequiredFieldValidator>

<cc1:CascadingDropDownID="cddlTest"runat="server"Category="Test"

                   LoadingText="Loading.."PromptText="-- Select --"  ServiceMethod="GetTestData"

                   ServicePath="~/ddlTestData.asmx"TargetControlID="ddlTest">

               </cc1:CascadingDropDown>