CannotUndoException while undoing textpane

[jogendar]

i created textpane and added undo manager like this...

final JTextPane jta;
    jta = new JTextPane();
    jta.setEditable(true);
    final UndoManager manager = new UndoManager();
    jta.getDocument().addUndoableEditListener(manager);

and undo button was coded as

ImageIcon undo = new ImageIcon(getClass().getResource("images/undo.png"));
	final JButton bundo = new JButton(undo);
    bundo.setActionCommand("undo");
	bundo.setToolTipText("undo");
    ActionListener act_undo = new ActionListener() {
    public void actionPerformed(ActionEvent e) {
       try {
            manager.undo();
         } 
         catch (CannotUndoException ue) {
            Toolkit.getDefaultToolkit().beep();
         }
      }
    };
    bundo.addActionListener(act_undo);

when i run this CannotUndoException was caught and i am unable to undo the textpane..
anyone help me