How to Validate CheckBoxList

I have trouble for validating a  CheckBoxList  control. I want  to force a  user
check the control before submitting the form. I've tried to use
Requiredfieldvalidator and  ControlToValidate but ASP.NET always
throw an error . 
Control 'boxid' referenced by the ControlToValidate property of
'boxid_required' cannot be validated.
 Pleas help me ! I am stack

The script is :
<asp:CheckBoxList id="boxid" runat="server" >
                       
<asp:ListItem  Text="A" runat="server" />
                       
<asp:ListItem  Text="B" runat="server" />
            &nb ...