Updatepanel in masterpage, trigger the FileUpload in a child webform-Help

I try to pass the postbacktrigger in the child webform using this code:

protected void Page_Load(object sender, EventArgs e)
{
//For file upload to be triggers
UpdatePanel updatepanel = Page.Master.FindControl("UpdatePanel1") as UpdatePanel;
UpdatePanelControlTrigger trigger = new PostBackTrigger();
trigger.ControlID = FileUpload1.UniqueID;
updatepanel.Triggers.Add(trigger);
}

But I got this error:
"Control with ID 'FileUpload1' being registered through RegisterAsyncPostBackControl
or RegisterPostBackControl must implement either INamingContainer, IPostBackDataHandler,
or IPostBackEventHandler."

Masterpage design source:
<form id="form1" runat="server">
<div>
<asp:ScriptManager ID="ScriptManager1" runat="server"></asp:ScriptManager>
<asp:Button ID="Button1" runat="server" Text="Button" OnClick="Button1_Click" />

<asp:UpdateProgress runat="server">
<ProgressTemplate>
<div class="upmodal">
<div class="upcenter">
Please wait... processing...
<img alt="" src="Loading.GIF" />
</div>
</div>
</ProgressTemplate>
</asp:UpdateProgress>

<asp:UpdatePanel ID ="UpdatePanel1" runat="server" UpdateMode ="Conditional" >
<Triggers >
<asp:AsyncPostBackTrigger ControlID ="Button1" />
</Triggers>
<ContentTemplate>

<asp:ContentPlaceHolder ID="ContentPlaceHolder1" runat="server">
</asp:ContentPlaceHolder>
</ContentTemplate>
</asp:UpdatePanel>

</div>
</form>


Child Webform Design source:
<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" runat="server">
<asp:FileUpload ID="FileUpload1" runat ="server" AllowMultiple ="true" />
<asp:Button ID ="btnUpload" Text ="Upload it" runat ="server" OnClick ="Click_Upload" />
<asp:Label ID="Label1" runat ="server" ></asp:Label>
</asp:Content>

Please Help Thanks