Button In Modal Panel Not Firing On First Click (Firing Second click)

HI,

I have a Modal Panel that is displayed on the click of a button. On the Modal Panel, I have another button used to search and return details to a gridview. The problem is that on the first click of the search button, nothing happens, but upon clicking it again, it works. Any ideas?

<asp:Panel ID="ModalPanel" runat="server" Width="700px" CssClass="PopupBox" DropShadow="true" CancelControlID="btnSearchCancel" OKControlID="btnSearchCancel0" Height="600px"><h2>Search For Details</h2><p><label>Search Field</label><asp:TextBox ID="tSearchField" runat="server"></asp:TextBox></p><p><asp:Button ID="btnSearchPopup" runat="server" Text="Search" CssClass="buttonCenter" 
                    onClick="btnSearchDetails_Click" CausesValidation="False"/><asp:Button ID="btnSearchCancel" runat="server" Text="Cancel" 
                    CausesValidation="False" /></p><asp:GridView ID="gvSearch" runat="server" AutoGenerateColumns="False" 
                DataKeyNames="RegAn" AllowPaging="true" CssClass="table-searchresults"></asp:GridView></asp:Panel><p><asp:ModalPopupExtender ID="mpeSearch" runat="server" PopupControlID="ModalPanel" 
                TargetControlID="btnSearch" /></p>

btnSearch_Details is some serverside code to fill the Gridview

Thanks in advance.