Problemas com ComboBox

import java.awt.BorderLayout;
import java.awt.Container;
import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.JComboBox;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JTextField;

public class Tela extends JFrame implements ActionListener{
	
	private JComboBox combo;
	private JTextField campo;
	
	public Tela(){
		super("Minha Tela");
		setSize(400, 400);
		setVisible(true);
		Container con = getContentPane();
		con.add(getPanel(), BorderLayout.CENTER);
		
		pack();
		setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
	}
	
	private JPanel getPanel(){
		JPanel painel = new JPanel();
		painel.setLayout(new GridLayout(2,1));
		Object objs[] = new Object[2];
		objs[0] = "Sim";
		objs[1] = "Não";
		combo = new JComboBox(objs);
		combo.addActionListener(this);
		painel.add(combo);
		campo = new JTextField();
		campo.setEnabled(false);
		painel.add(campo);
		
		return painel;
	}
	
	public static void main(String args[]){
		new Tela();
	}

	public void actionPerformed(ActionEvent e) {
		
		Object eventSource = e.getSource();
		
		if(eventSource instanceof JComboBox){
			JComboBox combo2 = (JComboBox)eventSource;
			System.out.println(combo2.getSelectedItem().toString());
			if(combo2.getSelectedItem().toString().equalsIgnoreCase("Não")){
				campo.setText("Nenhum");
				campo.setEnabled(true);
			}
			else{
				campo.setText("");
				campo.setEnabled(false);
			}
		}
	}
}