问ksoap2序列化错误

EN

public class MainActivity extends Activity { 

private static final String SOAP_ACTION = "http://tempuri.org/devuelveString"; //Variables
private static final String METHOD_NAME = "devuelveString";
private static final String NAMESPACE = "http://tempuri.org/";
private static final String URL = "http://localhost:52335/Service1.asmx";

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    Button boton = (Button) findViewById(R.id.Bboton);

    boton.setOnClickListener(new OnClickListener() {

        @Override
        public void onClick(View v) {

            InvocarWs(); //call the method

        }
    });
}

public void InvocarWs() {
    System.out.println("llama al metodo");
    try {

        SoapObject request = new SoapObject(NAMESPACE, METHOD_NAME);

        final SoapSerializationEnvelope envelope = new SoapSerializationEnvelope(
                SoapEnvelope.VER11); // utilizar la version que
                                        // corresponda:11 o 12
        envelope.dotNet = true; // para WS ASMX, sólo si fue construido con
                                // .Net
        envelope.setOutputSoapObject(request);

        HttpTransportSE transporte = new HttpTransportSE(URL);

        transporte.call(SOAP_ACTION, envelope);

        SoapPrimitive resultado = (SoapPrimitive) envelope.getResponse();

        System.out.println("" + resultado.toString());

    } catch (Exception e) {
        System.out.println("catch");
    }
}

04-26 08:03:08.731: E/AndroidRuntime(536): FATAL EXCEPTION: main
04-26 08:03:08.731: E/AndroidRuntime(536): java.lang.NoClassDefFoundError: org.ksoap2.serialization.SoapObject
04-26 08:03:08.731: E/AndroidRuntime(536):  at com.example.prueba.Conexion.InvocarWs(Conexion.java:24)
04-26 08:03:08.731: E/AndroidRuntime(536):  at com.example.prueba.MainActivity$1.onClick(MainActivity.java:25)
04-26 08:03:08.731: E/AndroidRuntime(536):  at android.view.View.performClick(View.java:2485)
04-26 08:03:08.731: E/AndroidRuntime(536):  at android.view.View$PerformClick.run(View.java:9080)
04-26 08:03:08.731: E/AndroidRuntime(536):  at android.os.Handler.handleCallback(Handler.java:587)
04-26 08:03:08.731: E/AndroidRuntime(536):  at android.os.Handler.dispatchMessage(Handler.java:92)
04-26 08:03:08.731: E/AndroidRuntime(536):  at android.os.Looper.loop(Looper.java:123)
04-26 08:03:08.731: E/AndroidRuntime(536):  at android.app.ActivityThread.main(ActivityThread.java:3683)
04-26 08:03:08.731: E/AndroidRuntime(536):  at java.lang.reflect.Method.invokeNative(Native Method)
04-26 08:03:08.731: E/AndroidRuntime(536):  at java.lang.reflect.Method.invoke(Method.java:507)
04-26 08:03:08.731: E/AndroidRuntime(536):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)
04-26 08:03:08.731: E/AndroidRuntime(536):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
04-26 08:03:08.731: E/AndroidRuntime(536):  at dalvik.system.NativeStart.main(Native Method)