ASP.Net - C# Checkbox All ครับ ผมต้องการให้มัน เลือกทั้งหมดได้แต่มันขึ้น Script Error

<script type="text/jscript">
    function ClickCheckAll(vol)
    {
    var i=1;
    for(i=1;i<=document.hdnCount.value;i++)
    {
        if(vol.checked == true)
        {
            eval("document.frmMain.chkDel"+i+".checked=true");
        }
        else
        {
            eval("document.frmMain.chkDel"+i+".checked=false");
        }
    }
    }
    function onDelete()
    {
        if(confirm('Do you want to delete ?')==true)
        {
            return true;
        }
        else
        {
            return false;
        }
    }
</script>

<asp:GridView ID="gvStaff" runat="server" Visible="False" AutoGenerateColumns="false" Width="100%" 
        BackColor="LightGoldenrodYellow" BorderColor="Tan" BorderWidth="1px" 
        CellPadding="2" ForeColor="Black" GridLines="None" >
        <AlternatingRowStyle BackColor="PaleGoldenrod" />
        <FooterStyle BackColor="Tan" />
        <HeaderStyle BackColor="Tan" Font-Bold="True" />
        <PagerStyle BackColor="PaleGoldenrod" ForeColor="DarkSlateBlue" 
            HorizontalAlign="Center" />
        <SelectedRowStyle BackColor="DarkSlateBlue" ForeColor="GhostWhite" />
        <SortedAscendingCellStyle BackColor="#FAFAE7" />
        <SortedAscendingHeaderStyle BackColor="#DAC09E" />
        <SortedDescendingCellStyle BackColor="#E1DB9C" />
        <SortedDescendingHeaderStyle BackColor="#C2A47B" />

        <Columns>

            <asp:TemplateField>
                <HeaderTemplate>
                    <asp:CheckBox ID="CheckAll" runat="server" Text="" OnClick="ClickCheckAll(this)" Width="90px" />
                </HeaderTemplate>
            <HeaderStyle HorizontalAlign="Center" />
            <ItemStyle HorizontalAlign="Center" />
            <EditItemTemplate>
                      
            </EditItemTemplate>
                <ItemTemplate>
                    <asp:CheckBox ID="chkObjective" runat="server" OnClick="select(this)" />
                </ItemTemplate>
            </asp:TemplateField>     
              
           <asp:BoundField DataField ="CITIZEN_ID" HeaderText="CITIZEN_ID" SortExpression="CITIZEN_ID" ItemStyle-HorizontalAlign="Center" />
           <asp:BoundField DataField ="STF_FNAME" HeaderText="Name" SortExpression="STF_FNAME" ItemStyle-HorizontalAlign="Center" />
           <asp:BoundField DataField ="STF_LNAME" HeaderText="Lastname" SortExpression="STF_LNAME" ItemStyle-HorizontalAlign="Center" />
           <asp:BoundField DataField ="BIRTHDAY" HeaderText="Birthday" SortExpression="BIRTHDAY" ItemStyle-HorizontalAlign="Center" />
           <asp:BoundField DataField ="TELEPHONE" HeaderText="Tel" SortExpression="TELEPHONE" ItemStyle-HorizontalAlign="Center" />
           
        </Columns>

    </asp:GridView>
<asp:Button ID="btnSubmit" runat="server" Text="Approve" />