validation errr

<tr> <td> <asp:Label ID="lblBlockType" AutoPostBack="true" runat="server" Text="Block Type" Visible="false"> </asp:Label> </td> <td> <asp:DropDownList ID="drpBlockType" runat="server" AutoPostBack="True" OnSelectedIndexChanged="drpBlockType_SelectedIndexChanged"> <asp:ListItem Text="--Select--" Value=""></asp:ListItem> <asp:ListItem Text="Domestic Block" Value="1"></asp:ListItem> <asp:ListItem Text="Overseas Block" Value="2"></asp:ListItem> </asp:DropDownList> &nbsp;<asp:Label ID="lblTypeRequired" runat="server" Text="*" ForeColor="Red" Visible="false"></asp:Label> <asp:RequiredFieldValidator ID="rfvBlockType" runat="server" ErrorMessage="Please Select Block type" ControlToValidate="drpBlockType" ValidationGroup="LeaveDetail" InitialValue="--Select--" Display="none"></asp:RequiredFieldValidator> <cc1:ValidatorCalloutExtender ID="vc ...