UNDO Problem

Checking the online demo it seems that the undo function works nice.
Unfortunately, I can't make it work in the following simple example:

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">

<html>
<head>
  <script type="text/javascript" src="../FCKeditor/fckeditor.js"></script>
  <script type="text/javascript">
      window.onload = function()
      {
        var oFCKeditor = new FCKeditor( 'Content' ) ;
        oFCKeditor.BasePath   = '../FCKeditor/';
        oFCKeditor.Width = '80%';
        oFCKeditor.ReplaceTextarea() ;
      }
    </script>
</head>
<body>
<textarea id="Content" name="Content">This is &lt;b&gt;the&lt;/b&gt;
initial value.</textarea>
</body>
</html>

Most of the times the undo button looks disabled and when it is enabled it doesn't work. Could anyone help?

Thank you,
Bill