Erro no ExpandableListView

public class ListaTeste extends Activity {
	List<Contato> listaContatos;
	public ExpandableListView ls;

	static final String[] tipoCalculos1 = new String[] { "Regra de 3 simples",
			"Fator de Potência" };

	public void onCreate(Bundle savedInstanceState) {

		super.onCreate(savedInstanceState);
		setContentView(R.layout.lista_teste);

		ArrayAdapter<String> adapter = new ArrayAdapter<String>(this,
				android.R.layout.simple_dropdown_item_1line, tipoCalculos1);
		ls = (ExpandableListView) findViewById(R.id.expandableListView1);
		ls.setAdapter(adapter);

	}

}