Эта реализация DialogFragment вызывает

IllegalStateException ("Вы не можете установить диалогs OnCancelListener или OnDismissListener ")

, Зачем? Решение?

public class OkCThreadDialog1 extends DialogFragment{

DialogInterface.OnCancelListener onCancelListener;

public OkCThreadDialog1(){
}

public static OkCThreadDialog1 newInstance(String title, String message) {
    OkCThreadDialog1 frag = new OkCThreadDialog1();
    Bundle args = new Bundle();
    args.putString("title", title);
    args.putString("message", message);
    frag.setArguments(args);
    return frag;
}


public Dialog onCreateDialog(Bundle savedInstanceState){

    AlertDialog.Builder builder = new AlertDialog.Builder(getActivity());


    builder .setTitle(getArguments().getString("title"))
            .setMessage(getArguments().getString("message"))
            .setOnCancelListener(onCancelListener)
            .setPositiveButton("Ok", new DialogInterface.OnClickListener() {
                public void onClick(DialogInterface dialog, int id) {
                }})
            .setNegativeButton("Cancel", new DialogInterface.OnClickListener() {
                public void onClick(DialogInterface dialog, int id) {
                    getDialog().cancel();
                }});

    return builder.create();
}

@Override
public void onAttach(Activity activity) {
    super.onAttach(activity);
    // Verify that the host activity implements the callback interface
    try {
        onCancelListener = (DialogInterface.OnCancelListener) activity;
    } catch (ClassCastException e) {
        // The activity doesn't implement the interface, throw exception
        throw new ClassCastException(activity.toString()
                + " must implement OkCancelDialogListener");
    }
}
}

Моя активность реализуетDialogInterface.OnCancelListener следующим образом:

public class MainActivity extends Activity implements OkCancelDialogListener{

static final String TAG ="MainActivity";

@Override
public void onCancel(DialogInterface dialog) {


}
}

Исключение выбрасывается изbuilder.create();, Какие'S не так?