How to add the Require field validation in dropDownCheckBoxes ...?

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C#

ASP.NET

Hi,

How to add the Require field validation in dropDownCheckBoxes ...?

This is my code

<asp:dropDownCheckBoxes ID="ddllanguage" runat="server" xmlns:asp="#unknown"  onchange="onSelectionChange()"
                AddJQueryReference="True" UseButtons="false" UseSelectAllNode="false" ClientIDMode="Static">
                <Style SelectBoxWidth="200" dropDownBoxBoxWidth="200" dropDownBoxBoxHeight="130" />
                <Texts SelectBoxCaption="-Select-" />
            </asp:dropDownCheckBoxes>
<asp:RequiredFieldValidator ID="RequiredFieldValidator2" runat="server"  ControlToValidate="ddllanguage"
                          ErrorMessage="*" ToolTip="Language is required"
                         ValidationGroup="Veterinary" InitialValue="-Select-"
                        ></asp:RequiredFieldValidator>

When i run the application i am getting this error "Control 'ddllanguage' referenced by the ControlToValidate property of 'RequiredFieldValidator2' cannot be validated."

anybody give me any suggestion how to add the validation....

Regrads
Ravinder