AsyncFileUpload1.Enabled = true not functioning well

I have few controls in a update panel and I had set all the controls enabled=false when Page_Load. I have one button to set all the controls enabled=true when onclick. There is a problem with one of the control which is AsyncFileUpload. First click all the controls was enabled except AsyncFileUpload. AsyncFileUpload only enabled if I click the same button twice. This is happen consistently. 

<form id="form1" runat="server"><div><asp:ScriptManager ID="ScriptManager1" runat="server"></asp:ScriptManager><cc1:AsyncFileUpload runat="server" ID="AsyncFileUpload1" /><asp:Button ID="Button1" runat="server" Text="Button" /></div></form>

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

        If Page.IsPostBack = False Then
            Me.AsyncFileUpload1.Enabled = False
        End If

    End Sub

    Protected Sub Button1_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles Button1.Click
        Me.AsyncFileUpload1.Enabled = True
    End Sub