requiredfieldvalidator in modalpop prevent modalpopup from showing

hello experts;

I have the following requiredfieldvalidator code below. when i add it to the panel, my modalpopup doesn't show up anymore. please help. i am using visual studio 2008 framework 3.5

<asp:RequiredFieldValidator ID = "reqddlstarthh"
runat = "server"
ControlToValidate = "ddlstarthh"
InitialValue= ""
Display = "Dynamic"
ErrorMessage = "Please pick a start hh">
</asp:RequiredFieldValidator>

<asp:Button ID="HiddenTargetControlForModalPopup" runat="server" Style="display: none" />
<ccl:ModalPopupExtender ID="ModalPopupExtender_btnnewdayshift" runat="server" PopupControlID="PNL"
CancelControlID="imgbtnmodalpopupdayclose" BackgroundCssClass="modalpopupbackground"
TargetControlID="HiddenTargetControlForModalPopup">
</ccl:ModalPopupExtender>
<asp:Panel ID="PNL" runat="server" CssClass="modalPopup" Height="200px" Width="300px"
Style="display: none">

<asp:DropDownList ID="ddlstarthh" runat="server">
<asp:ListItem Text = "" Value=""></asp:ListItem>
<asp:ListItem Value="01">1</asp:ListItem>
<asp:ListItem Value="02">2</asp:ListItem>
<asp:ListItem Value="03">3</asp:ListItem>
<asp:ListItem Value="04">4</asp:ListItem>
<asp:ListItem Value="05">5</asp:ListItem>
<asp:ListItem Value="06">6</asp:ListItem>
<asp:ListItem Value="07">7</asp:ListItem>
<asp:ListItem Value="08">8</asp:ListItem>
<asp:ListItem Value="09">9</asp:ListItem>
<asp:ListItem Value="10">10</asp:ListItem>
<asp:ListItem Value="11">11</asp:ListItem>
<asp:ListItem Value="12">12</asp:ListItem>
</asp:DropDownList>

</panel>