RadioButtonList单选和RequiredFieldValidator验证是否选中

<asp:RadioButtonList ID="Radio2" RepeatDirection="Horizontal" runat="server">
<asp:ListItem>@163.com</asp:ListItem>
<asp:ListItem>@126.com</asp:ListItem>
<asp:ListItem>@qq.com</asp:ListItem>
</asp:RadioButtonList> 
<asp:RequiredFieldValidator ID="RequiredFieldValidator1" runat="server" ControlToValidate="Radio2" ErrorMessage="请选择接收邮箱"></asp:RequiredFieldValidator>