GridView SelectedIndexChanged event error: Argument Out of Range exception

Hi Guys,

I'm facing error in code behind. It is showing the above exception. Code behind code is as follows

 HTML

<asp:GridView ID="gv_TPBill" runat="server" AutoGenerateColumns="false" DataKeyNames="Instance Name"
    OnRowDataBound="OnRowDataBound" OnSelectedIndexChanged="OnSelectedIndexChanged">
    <Columns>
        <asp:TemplateField HeaderText="Client Name" ItemStyle-Width="150">
            <ItemTemplate>
                <asp:Label ID="lblCName" runat="server" Text='<%# Eval("Client Name") %>'></asp:Label>
            </ItemTemplate>
        </asp:TemplateField>
        <asp:TemplateField HeaderText="Instance Name" ItemStyle-Width="150">
            <ItemTemplate>
                <asp:Label ID="lblIName" runat="server" Text='<%# Eval("Instance Name") %>'></asp:Label>
            </ItemTemplate>
        </asp:TemplateField>
        <asp:CommandField ButtonType="Image" ShowSelectButton="true" HeaderText="View Files"
            SelectImageUrl="~/Images/view.png" ItemStyle-Width="150" />
    </Columns>
</asp:GridView>

Code

protected void OnSelectedIndexChanged(object sender, EventArgs e)
{
    string Clabel = ((Label)gv_TPBill.SelectedRow.FindControl("lblCName")).Text;
    string Ilabel = ((Label)gv_TPBill.SelectedRow.FindControl("lblIName")).Text;
    string directoryPath = Server.MapPath(string.Format("~/D:\\TrackPoint App UAT\\TrackPointBilling/{0}/", Clabel));
    if (!Directory.Exists(directoryPath))
    {
        Directory.CreateDirectory(directoryPath);
        Response.Redirect("~/Upload.aspx?IName=" + Ilabel + "&CName=" + Clabel);
    }
    else
    {
        Response.Redirect("~/Upload.aspx?IName=" + Ilabel + "&CName=" + Clabel);
    }
}

 Please help