A control with ID 'chkEmp' could not be found for the trigger in UpdatePanel 'UpdatePanel2'

Hi

I keep getting the above error when loading up the page.  Please see my source code:

<asp:UpdatePanel ID="UpdatePanel2" runat="server"><ContentTemplate><div class="row"><div class="col-lg-2">Filter bookings by type:</div><div class="col-lg-4"><asp:DropDownList ID="ddlBookingType" runat="server" onselectedindexchanged="ddlBookingType_SelectedIndexChanged" AutoPostBack="True" class="form-control"><asp:ListItem Value="6" Selected="True">Choose...</asp:ListItem><asp:ListItem Value="0">All Bookings</asp:ListItem><asp:ListItem Value="1">New Bookings</asp:ListItem><asp:ListItem Value="2">Approved Bookings</asp:ListItem><asp:ListItem Value="3">Completed Bookings</asp:ListItem><asp:ListItem Value="4">Deleted Bookings</asp:ListItem><asp:ListItem Value="5">Future Bookings</asp:ListItem></asp:DropDownList></div><div class="col-lg-6"></div></div><div class="row"><div class="col-lg-2"></div><div class="col-lg-10"><p><asp:GridView ID="grdvBookings" ClientIDMode="Static" runat="server" CssClass="gridview" AllowSorting="True" AutoGenerateColumns="False" AllowPaging="True" DataKeyNames="BookingID,BookingRef" CellPadding="10" GridLines="None" ><PagerSettings PageButtonCount="10" /><PagerStyle cssClass="Test"/><RowStyle BorderStyle="None" BackColor="#FFFFFF" HorizontalAlign="Center" ForeColor="#333333"></RowStyle><Columns><asp:HyperLinkField Text="Click for more info"  DataNavigateUrlFormatString="~/Admin/Bookings/BookingDetails.aspx?BookingId={0}" HeaderText="Go To Booking" DataNavigateUrlFields="BookingRef" /><asp:BoundField HeaderText="Booking Ref" Visible="True" DataField="BookingRef" ReadOnly="true" ></asp:BoundField><asp:BoundField HeaderText="Name" DataField="Name" ReadOnly="true" ></asp:BoundField><asp:BoundField HeaderText="Booking Time" DataField="BookingTime" ReadOnly="true" ></asp:BoundField><asp:BoundField DataField="EmailAddress" HeaderText="EmailAddress" ReadOnly="true" ></asp:BoundField><asp:BoundField DataField="TelNo" HeaderText="Tel No" ReadOnly="true" ></asp:BoundField><asp:BoundField DataField="Communication Method" HeaderText="Contact Method" ReadOnly="true" ></asp:BoundField><asp:BoundField DataField="Current Status" HeaderText="Booking Status" ReadOnly="true" ></asp:BoundField><asp:BoundField DataField="Technician" HeaderText="Technician" NullDisplayText="No Technician Assigned" ReadOnly="True" ></asp:BoundField><asp:TemplateField ItemStyle-Width="40px"><ItemStyle HorizontalAlign="Center" VerticalAlign="Middle" /><%----%><HeaderTemplate><asp:CheckBox ID="SelectCheckBox" runat="server" ClientIDMode="Static" onclick="CheckAllEmp(this);" /></HeaderTemplate><ItemTemplate><asp:CheckBox ID="chkEmp" runat="server" oncheckedchanged="chkEmp_CheckedChanged"></asp:CheckBox></ItemTemplate><ItemStyle HorizontalAlign="Center" VerticalAlign="Middle" /></asp:TemplateField></Columns><EditRowStyle BorderStyle="None" Width="100px" BackColor="#999999" /><EmptyDataTemplate>There is no data visible</EmptyDataTemplate><FooterStyle BackColor="#CC6699" Height="40px" Font-Bold="True" ForeColor="White" /><HeaderStyle BackColor="#CC6699" Height="40px" Font-Bold="True" ForeColor="White" /><AlternatingRowStyle BorderStyle="None" BackColor="#F5F5F5" ForeColor="#284775" /><SelectedRowStyle BackColor="#E2DED6" Font-Bold="True" ForeColor="#333333" /><SortedAscendingCellStyle BackColor="#E9E7E2" /><SortedAscendingHeaderStyle BackColor="#506C8C" /><SortedDescendingCellStyle BackColor="#FFFDF8" /><SortedDescendingHeaderStyle BackColor="#6F8DAE" /></asp:GridView></p></div></div><div class="row"><div class="col-lg-12 col-md-12 col-sm-12"><p class="text-center"><asp:Button ID="btnNew" runat="server" Enabled="false" Text="Mark New" CssClass="btn btn-left" onclick="MarkNew_Click" /><asp:Button ID="btnApproved" runat="server" Enabled="false"  Text="Mark Approve" CssClass="btn btn-left" onclick="MarkApproved_Click" /><asp:Button ID="btnCompleted" runat="server" Enabled="false" Text="Mark Complete" CssClass="btn btn-left" onclick="MarkCompleted_Click" /><asp:Button ID="btnDeleted" runat="server" Enabled="false" Text="Mark Delete" CssClass="btn btn-left" onclick="MarkDeleted_Click" /></p></div></div></ContentTemplate><Triggers><asp:AsyncPostBackTrigger ControlID="ddlBookingType" EventName="SelectedIndexChanged" /><asp:AsyncPostBackTrigger ControlID="chkEmp" EventName="CheckedChanged" /><asp:AsyncPostBackTrigger ControlID="btnNew" EventName="Click" /><asp:AsyncPostBackTrigger ControlID="btnApproved" EventName="Click" /><asp:AsyncPostBackTrigger ControlID="btnCompleted" EventName="Click" /><asp:AsyncPostBackTrigger ControlID="btnDeleted" EventName="Click" /></Triggers></asp:UpdatePanel>

I want it to call this function:

protected void chkEmp_CheckedChanged(object sender, EventArgs e)
    {
        CheckBox chkTest = (CheckBox)sender;
        GridViewRow grdRow = (GridViewRow)chkTest.NamingContainer;
        int count = 0;

        foreach (GridViewRow gvrow in grdvBookings.Rows)
        {
            CheckBox chk = (CheckBox)gvrow.FindControl("chkEmp");

            if (chk.Checked)
            {
                count++;
            }
        }

        if (count == 1)
        {
            btnNew.Enabled = true;
            btnApproved.Enabled = true;
            btnCompleted.Enabled = true;
            btnDeleted.Enabled = true;
        }
        else
        {
            btnNew.Enabled = false;
            btnApproved.Enabled = false;
            btnCompleted.Enabled = false;
            btnDeleted.Enabled = false;
        }
    }

Any ideas on how to get around it?

Thanks

Rob