UpdatePanel asynchronous postback not calling TextChanged event of TextBox
<p>Why isn't the UpdatePanel asynchronous postback calling the TextChanged event of the TextBox, in the code shown below?</p>

<pre><code>&lt;%@ Page Title="" Language="VB" MasterPageFile="~/Master.master" CodeFile="Default2.aspx.vb" Inherits="Default2" AutoEventWireup="false" %&gt;
&lt;asp:Content ID="Content1" ContentPlaceHolderID="cphHead" runat="Server"&gt;
&lt;/asp:Content&gt;
&lt;asp:Content ID="Content2" ContentPlaceHolderID="ConPhameNet" runat="Server"&gt;
    &lt;asp:UpdatePanel ID="UpdatePanel1" runat="server" UpdateMode="Conditional"&gt;
        &lt;ContentTemplate&gt;
            &lt;asp:TextBox ID="TextBox1" runat="server" AutoPostBack="True"&gt;&lt;/asp:TextBox&gt;
            &lt;asp:Label ID="Labelabc1" runat="server" Text="Label"&gt;&lt;/asp:Label&gt;
        &lt;/ContentTemplate&gt;
        &lt;Triggers&gt;
            &lt;asp:AsyncPostBackTrigger ControlID="TextBox1" EventName="TextChanged" /&gt;
        &lt;/Triggers&gt;
    &lt;/asp:UpdatePanel&gt;
&lt;/asp:Content&gt;
</code></pre>