按下按钮时，ASP dropdownlist selecedvalue为空白

嗨，我的下拉列表有问题。我已经走了，但似乎没有人遇到过同样的问题。我有一个带有控制面板的updatepanel。单击按钮时，我想从控件中获取值，问题是单击按钮时，selected值始终为空白。

这是HTML

<asp:UpdatePanel runat="server" ChildrenAsTriggers="true" ID="updpnl1" EnableViewState="False">
                <ContentTemplate>
                    <asp:Panel runat="server" ID="pnlLSP" CssClass="textBox" Width="85%" Visible="False">
                        <div>
                                    <asp:DropDownList ID="ddlLSPHours" runat="server" ValidationGroup="LSP" 
                                EnableViewState="True">
                                </asp:DropDownList>
                                <asp:RequiredFieldValidator ID="RequiredFieldValidator8" runat="server" 
                                    ControlToValidate="ddlLSPHours" ErrorMessage="RequiredFieldValidator" 
                                    ValidationGroup="LSP">*</asp:RequiredFieldValidator>
                                &nbsp;<asp:DropDownList ID="ddlLSPMins" runat="server" ValidationGroup="LSP" EnableViewState="True">
                                </asp:DropDownList>
                                <asp:RequiredFieldValidator ID="RequiredFieldValidator9" runat="server" 
                                    ControlToValidate="ddlLSPMins" ErrorMessage="RequiredFieldValidator" 
                                    ValidationGroup="LSP">*</asp:RequiredFieldValidator>
                        </div>
                        <div style="text-align: center">
                            <asp:Button ID="btnLSPDone" runat="server" CssClass="button" Text="Done" Style="margin-top: 10px"
                                Width="100px" ValidationGroup="LSP" OnClick="btnLSPDone_Click" />
                        </div>
                    </asp:Panel>
                </ContentTemplate>
                <Triggers>
                    <asp:PostBackTrigger ControlID="btnLSPDone"></asp:PostBackTrigger>
                </Triggers>
            </asp:UpdatePanel>

用于填充下拉列表的C＃代码

protected void Page_Load(object sender, EventArgs e)
{
    if (!Page.IsPostBack)
    {
        //populate hours ddl
        for (int i = 0; i < 25; i++)
        {
            string number = i.ToString();

            if (i < 10)
            {
                number = "0" + i.ToString();
            }

            ddlLSPHours.Items.Add(number)
        }

        //populate mins ddl
        for (int i = 0; i < 61; i++)
        {
            string number = i.ToString();

            if (i < 10)
            {
                number = "0" + i.ToString();
            }

            ddlLSPMins.Items.Add(number);
        }
    }
}

protected void btnLSPDone_Click(object sender, EventArgs e)
{
    string timeSelected = ddlLSPHours.SelectedValue + ":" + ddlLSPMins.SelectedValue;
}