[asp.net ajax]UpdatePanelとPopupControlExtenderの内部のButtonのClickが呼ばれない

<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="cc1" %>

<asp:ScriptManager ID="ScriptManager1" runat="server" />



<asp:UpdatePanel ID="UpdatePanel2" runat="server">

    <ContentTemplate >

        <asp:TextBox ID="TextBox1" runat="server" Width="150px" />

        <cc1:PopupControlExtender ID="PopupControlExtender1" runat="server" TargetControlID="TextBox1"

            PopupControlID="pnl1" Position="Bottom" />

        <asp:Panel runat="server" id="pnl1">

            <asp:RadioButtonList ID="RadioButtonList1" runat="server" AutoPostBack="True"

                Width="150px" OnSelectedIndexChanged="RadioButtonList1_SelectedIndexChanged" Height="20px">

                <asp:ListItem>1</asp:ListItem>

                <asp:ListItem>2</asp:ListItem>

            </asp:RadioButtonList>



            <asp:Button ID="Button1" runat="server" Text="Button" Width="150px" OnClick="Button1_Click" />



            <br /><asp:TextBox ID="TextBox3" runat="server" Width="150px" />

            <br /><asp:TextBox ID="TextBox2" runat="server" Width="150px" />

        </asp:Panel>

    </ContentTemplate>

</asp:UpdatePanel>

Shared count As Integer



Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load

    If Me.IsPostBack() Then

        count = count + 1

        TextBox3.Text = "count:" + count.ToString()

    Else

        count = 0

        TextBox3.Text = DateTime.Now()

    End If

End Sub



Protected Sub RadioButtonList1_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs)

    TextBox3.Text = "rad is pushed:" + RadioButtonList1.SelectedValue

    TextBox2.Text = String.Empty

    PopupControlExtender1.Commit(TextBox3.Text)

End Sub



Protected Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click

    TextBox2.Text = "pushed"

    PopupControlExtender1.Commit(TextBox3.Text)

End Sub