如何将面板公开为用户控件中的属性?

我有一个带有面板的用户控件:

自定义控件.ascx

<%@ Control Language="vb" CodeBehind="CustomControl.ascx.vb" %>
<asp:Panel ID="myPanel" runat="server"></asp:Panel>

我希望把这个面板作为一个我可以在设计时使用的属性来公开。

自定义控件.ascx.vb

<ParseChildren(True), PersistChildren(False)> _
Partial Public Class CustomControl
    Inherits System.Web.UI.UserControl

#Region "Members and Properties"
    <PersistenceMode(PersistenceMode.InnerProperty)> _
    Public Property SomePanel() As Panel
        Get
            Return myPanel
        End Get
        Set(ByVal value As Panel)
            myPanel= value
        End Set
    End Property

#End Region


    Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
         Response.Write(myPanel.HasControls.ToString())

    End Sub

End Class

然后将控件放到这样的页面上:

<ucl:CustomControl runat="server">
    <SomePanel>
        <asp:Label ID="lblText" AssociatedControlID="txtBox" runat="server"></asp:Label>
        <asp:TextBox id="txtBox" runat="server"></asp:TextBox>
    </SomePanel>
</ucl:CustomControl>

当我运行页面时,hasControls为true,但不呈现任何内容。我做错什么了?