Type 'AjaxControlToolkit.AccordionPane' does not have a public property named 'Panes'.

Hi,
What is the reason for the following error
Type 'AjaxControlToolkit.AccordionPane' does not have a public property named 'Panes'.

My code is as follow
<asp:ScriptManager ID="ScriptManager1" runat="server">
    </asp:ScriptManager>
<asp:UpdatePanel ID="UpdatePanel2" runat="server">
                <ContentTemplate>
                
                    <asp:AccordionPane ID="AccordionPane1" runat="server" SelectedIndex="0">
             ...