Why am I receiving Specified argument was out of the range of valid value error

<table class="taskGridView"  runat="server">
    <tr>
        <td>
            <asp:DropDownList ID="ddlTaskName" CssClass="chosen-select" DataSourceID="dsPopulateTaskName" AutoPostBack="true" DataValueField="Task Name" runat="server" Width="100%" Font-Size="11px" AppendDataBoundItems="true" OnSelectedIndexChanged="ddlTaskName_onSelectIndexChanged">
                <asp:ListItem Text="All" Value="%"></asp:ListItem>
            </asp:DropDownList>
            <asp:SqlDataSource ID="dsPopulateTaskName" runat="server" ConnectionString="<%$ ConnectionStrings:gvConnString %>" SelectCommand=""></asp:SqlDataSource>
        </td>
        <td>
            <asp:DropDownList ID="ddlService" CssClass="chosen-select" DataSourceID="dsPopulateService" AutoPostBack="true" DataValueField="Service" runat="server" Width="100%" Font-Size="11px" AppendDataBoundItems="true" OnSelectedIndexChanged="ddlService_onSelectIndexChanged">
                <asp:ListItem Text="All" Value="%"></asp:ListItem>
            </asp:DropDownList>
            <asp:SqlDataSource ID="dsPopulateService" runat="server" ConnectionString="<%$ ConnectionStrings:gvConnString %>" SelectCommand=""></asp:SqlDataSource>
        </td>
        <td>
            <asp:DropDownList ID="ddlStatus" CssClass="chosen-select" DataSourceID="dsPopulateStatus" AutoPostBack="true" DataValueField="Status" runat="server" Width="100%" Font-Size="11px" AppendDataBoundItems="true" OnSelectedIndexChanged="ddlStatus_onSelectIndexChanged">
                <asp:ListItem Text="All" Value="%"></asp:ListItem>
            </asp:DropDownList>
            <asp:SqlDataSource ID="dsPopulateStatus" runat="server" ConnectionString="<%$ ConnectionStrings:gvConnString %>" SelectCommand=""></asp:SqlDataSource>
        </td>
        <td>
            <asp:DropDownList ID="ddlDueDate" CssClass="chosen-select" DataSourceID="dsPopulateDueDate" AutoPostBack="true" DataValueField="Due Date" runat="server" Width="100%" Font-Size="11px" AppendDataBoundItems="true" OnSelectedIndexChanged="ddlDueDate_onSelectIndexChanged">
                <asp:ListItem Text="All" Value="%"></asp:ListItem>
            </asp:DropDownList>
            <asp:SqlDataSource ID="dsPopulateDueDate" runat="server" ConnectionString="<%$ ConnectionStrings:gvConnString %>" SelectCommand=""></asp:SqlDataSource>
        </td>
        <td>
            <asp:DropDownList ID="ddlOwner" CssClass="chosen-select" DataSourceID="dsPopulateOwner" AutoPostBack="true" DataValueField="Owner" runat="server" Width="100%" Font-Size="11px" AppendDataBoundItems="true" OnSelectedIndexChanged="ddlOwner_onSelectIndexChanged">
                <asp:ListItem Text="All" Value="%"></asp:ListItem>
            </asp:DropDownList>
            <asp:SqlDataSource ID="dsPopulateOwner" runat="server" ConnectionString="<%$ ConnectionStrings:gvConnString %>" SelectCommand=""></asp:SqlDataSource>
        </td>
        <td>
            <asp:DropDownList ID="ddlClient" CssClass="chosen-select" DataSourceID="dsPopulateClient" AutoPostBack="true" DataValueField="Client" runat="server" Width="100%" Font-Size="11px" AppendDataBoundItems="true" OnSelectedIndexChanged="ddlClient_onSelectIndexChanged">
                <asp:ListItem Text="All" Value="%"></asp:ListItem>
            </asp:DropDownList>
            <asp:SqlDataSource ID="dsPopulateClient" runat="server" ConnectionString="<%$ ConnectionStrings:gvConnString %>" SelectCommand=""></asp:SqlDataSource>
        </td>
        <td>
            <asp:DropDownList ID="ddlSite" CssClass="chosen-select" DataSourceID="dsPopulateSite" AutoPostBack="true" DataValueField="Site" runat="server" Width="100%" Font-Size="11px" AppendDataBoundItems="true" OnSelectedIndexChanged="ddlSite_onSelectIndexChanged">
                <asp:ListItem Text="All" Value="%"></asp:ListItem>
            </asp:DropDownList>
            <asp:SqlDataSource ID="dsPopulateSite" runat="server" ConnectionString="<%$ ConnectionStrings:gvConnString %>" SelectCommand=""></asp:SqlDataSource>
        </td>
        <td>
            <asp:DropDownList ID="ddlPractice" CssClass="chosen-select" DataSourceID="dsPopulatePractice" AutoPostBack="true" DataValueField="Practice" runat="server" Width="100%" Font-Size="11px" AppendDataBoundItems="true" OnSelectedIndexChanged="ddlPractice_onSelectIndexChanged">
                <asp:ListItem Text="All" Value="%"></asp:ListItem>
            </asp:DropDownList>
            <asp:SqlDataSource ID="dsPopulatePractice" runat="server" ConnectionString="<%$ ConnectionStrings:gvConnString %>" SelectCommand=""></asp:SqlDataSource>
        </td>
        <td>
            <asp:DropDownList ID="ddlProvider" CssClass="chosen-select" DataSourceID="dsPopulateProvider" AutoPostBack="true" DataValueField="Provider" runat="server" Width="100%" Font-Size="11px" AppendDataBoundItems="true" OnSelectedIndexChanged="ddlProvider_onSelectIndexChanged">
                <asp:ListItem Text="All" Value="%"></asp:ListItem>
            </asp:DropDownList>
            <asp:SqlDataSource ID="dsPopulateProvider" runat="server" ConnectionString="<%$ ConnectionStrings:gvConnString %>" SelectCommand=""></asp:SqlDataSource>
        </td>
    </tr>
</table>
<asp:GridView ShowHeaderWhenEmpty="false" AlternatingRowStyle-BackColor="#EBE9E9" AutoGenerateColumns="false" OnSorting="yourTasksGV_Sorting" AllowSorting="true" ID="yourTasksGV" runat="server" ClientIDMode="Static" EmptyDataText="You currently have no tasks assigned to you" OnRowDataBound="yourTasksGV_RowDataBound" OnRowCreated="yourTasksGV_RowCreated">
    <Columns>
        <asp:HyperLinkField Target="_self" DataNavigateUrlFields="Task Detail" DataTextField="Task Name" DataNavigateUrlFormatString="" HeaderText="Task Detail" SortExpression="Task Name" ItemStyle-CssClass="taskTableColumn" />
        <asp:BoundField DataField="Service" HeaderText="Service" SortExpression="Service" ItemStyle-CssClass="taskTableColumn" />
        <asp:BoundField DataField="Status" HeaderText="Status" SortExpression="Status" ItemStyle-CssClass="taskTableColumn" />
        <asp:BoundField DataField="Due Date" HeaderText="Due" SortExpression="Due Date" ItemStyle-CssClass="taskTableColumn" />
        <asp:BoundField DataField="Owner" HeaderText="Owner" SortExpression="Owner" ItemStyle-CssClass="taskTableColumn" />
        <asp:BoundField DataField="Client" HeaderText="Client" SortExpression="Client" ItemStyle-CssClass="taskTableColumn" />
        <asp:BoundField DataField="Site" HeaderText="Site" SortExpression="Site" ItemStyle-CssClass="taskTableColumn" />
        <asp:BoundField DataField="Practice" HeaderText="Practice" SortExpression="Practice" ItemStyle-CssClass="taskTableColumn" />
        <asp:BoundField DataField="Provider" HeaderText="Provider" SortExpression="Provider" ItemStyle-CssClass="taskTableColumn" />
    </Columns>
</asp:GridView>

protected void yourTasksGV_RowCreated(object sender, GridViewRowEventArgs e)
{
    if (e.Row.RowType == DataControlRowType.Header)
    {
        GridView hGrid = (GridView)sender;
        GridViewRow gvrRow = new GridViewRow(0, 0, DataControlRowType.Header, DataControlRowState.Insert);


        TableHeaderCell tcCellTask = new TableHeaderCell();
        tcCellTask.Controls.Add(ddlTaskName);
        gvrRow.Cells.Add(tcCellTask);
        yourTasksGV.Controls[0].Controls.AddAt(0, gvrRow);

        TableHeaderCell tcCellSvc = new TableHeaderCell();
        tcCellSvc.Controls.Add(ddlService);
        gvrRow.Cells.Add(tcCellSvc);
        yourTasksGV.Controls[0].Controls.AddAt(1, gvrRow);

        /*TableHeaderCell tcCellStats = new TableHeaderCell();
        tcCellStats.Controls.Add(ddlStatus);
        gvrRow.Cells.Add(tcCellStats);
        yourTasksGV.Controls[0].Controls.AddAt(2, gvrRow);

        TableHeaderCell tcCellDD = new TableHeaderCell();
        tcCellDD.Controls.Add(ddlDueDate);
        gvrRow.Cells.Add(tcCellDD);
        yourTasksGV.Controls[0].Controls.AddAt(3, gvrRow);

        TableHeaderCell tcCellOwner = new TableHeaderCell();
        tcCellOwner.Controls.Add(ddlOwner);
        gvrRow.Cells.Add(tcCellOwner);
        yourTasksGV.Controls[0].Controls.AddAt(4, gvrRow);

        TableHeaderCell tcCellCli = new TableHeaderCell();
        tcCellCli.Controls.Add(ddlClient);
        gvrRow.Cells.Add(tcCellCli);
        yourTasksGV.Controls[0].Controls.AddAt(5, gvrRow);

        TableHeaderCell tcCellSit = new TableHeaderCell();
        tcCellSit.Controls.Add(ddlSite);
        gvrRow.Cells.Add(tcCellSit);
        yourTasksGV.Controls[0].Controls.AddAt(6, gvrRow);

        TableHeaderCell tcCellPra = new TableHeaderCell();
        tcCellPra.Controls.Add(ddlPractice);
        gvrRow.Cells.Add(tcCellPra);
        yourTasksGV.Controls[0].Controls.AddAt(7, gvrRow);

        TableHeaderCell tcCellPro = new TableHeaderCell();
        tcCellPro.Controls.Add(ddlProvider);
        gvrRow.Cells.Add(tcCellPro);
        yourTasksGV.Controls[0].Controls.AddAt(8, gvrRow);*/

        LinkButton btnSort;
        System.Web.UI.WebControls.Image image;
        //iterate through all the header cells
        foreach (TableCell cell in e.Row.Cells)
        {
            //check if the header cell has any child controls
            if (cell.HasControls())
            {
                //get reference to the button column
                btnSort = (LinkButton)cell.Controls[0];
                image = new System.Web.UI.WebControls.Image();
                if (ViewState["sortExp"] != null)
                {
                    //see if the button user clicked on and the sortexpression in the viewstate are same
                    //this check is needed to figure out whether to add the image to this heade column or not
                    if (btnSort.CommandArgument == ViewState["sortExp"].ToString())
                    {
                        //check what type of sort order it is
                        if (ViewState["sortOrder"].ToString() == "Asc")
                        {
                            image.ImageUrl = "../theImages/up.png";
                            image.CssClass = "hdrImage";
                        }
                        else
                        {
                            image.ImageUrl = "../theImages/down.png";
                            image.CssClass = "hdrImage";
                        }
                        cell.Controls.Add(image);
                    }
                }
            }
        }
    }
}

After creating the first two header cell, when it gets to the third and consequent one, I get an error stating:

Specified argument was out of the range of valid values.

Parameter name: index

I have 9 columns in the GridView but I am not sure why I am receiving the error.

How do I resolve it?

If I uncomment everything, only three columns are created but I still get the same error.