OnTextChanged not firing

It really should be a simple one, but I haven't been able to make it work. After changing text in the txtColorPages TextBox, the event is not fired and the function CalculatePages never runs. Instead, the TextBox gets emptied every time.
<asp:UpdatePanel ID="pagesUP" runat="server" EnableViewState="true" UpdateMode="Always"><ContentTemplate> 
<asp:TextBox ID="txtColorPages" runat="server" OnTextChanged="CalculatePages" AutoPostBack="true" EnableViewState="true" />  <asp:Label ID="lblColorPages" runat="server"/>  <asp:Label ID="lblBWPages" runat="server"/>  </ContentTemplate>  <Triggers>  <asp:AsyncPostBackTrigger ControlID="txtColorPages" EventName="TextChanged" />  </Triggers>  </asp:UpdatePanel> Help would be appreciated.
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