ArcGIS Android Reverse geocoding, Cannot perform query. Invalid query parameters

I'm trying to write reverse geocoding. And i copied code given here... Here is my code sample..

      public class MagProActivity extends Activity {

            private MapView mMapView;
            private GraphicsLayer locationLayer; 


            /** Called when the activity is first created. */
            @Override
            public void onCreate(Bundle savedInstanceState) {
                super.onCreate(savedInstanceState);
                requestWindowFeature(Window.FEATURE_NO_TITLE);
                setContentView(R.layout.main);

                mMapView = (MapView) findViewById(R.id.map);
                mMapView.addLayer(new ArcGISDynamicMapServiceLayer("http://services.arcgisonline.com/ArcGIS/rest/services/World_Street_Map/MapServer"));

mMapView.setOnLongPressListener(new OnLongPressListener() {

            private static final long serialVersionUID = 1L;

            @Override
            public void onLongPress(float x, float y) {
                Point mapPoint = mMapView.toMapPoint(x, y);

                new ReverseGeocoding(MagProActivity.this, mMapView).execute(mapPoint);

            }
        });

      }
    }

ReverseGeocoding class

import java.util.Map;

import android.app.Activity;
import android.os.AsyncTask;
import android.widget.Toast;

import com.esri.android.map.MapView;
import com.esri.core.geometry.Point;
import com.esri.core.geometry.SpatialReference;
import com.esri.core.tasks.ags.geocode.Locator;
import com.esri.core.tasks.ags.geocode.LocatorReverseGeocodeResult;

public class ReverseGeocoding extends AsyncTask<Point, Void, LocatorReverseGeocodeResult>{

    private Activity curActivity;
    private MapView mapView;

    public ReverseGeocoding(Activity _curActivity, MapView _mapView){
        this.curActivity = _curActivity;
        this.mapView = _mapView;
    }

    @Override
    protected LocatorReverseGeocodeResult doInBackground(Point... params) {
        // create results object and set to null
        LocatorReverseGeocodeResult result = null;
        // set the geocode service
        Locator locator = new Locator();
        try {

            // Attempt to reverse geocode the point.
            // Our input and output spatial reference will be the same as the map.
            SpatialReference mapRef = mapView.getSpatialReference();
            result = locator.reverseGeocode(params[0], 50.0, mapRef, mapRef);
        } catch (Exception e) {
            e.printStackTrace();
        }
        // return the resulting point(s)
        return result;
    }

    protected void onPostExecute(LocatorReverseGeocodeResult result) {

        String resultAddress = "";

        // Construct a nicely formatted address from the results
        StringBuilder address = new StringBuilder();
        if (result != null && result.getAddressFields() != null) {
            Map addressFields = result.getAddressFields();
            address.append(String.format("%s\n%s, %s %s",
                    addressFields.get("Street"), addressFields.get("City"),
                    addressFields.get("State"), addressFields.get("ZIP")));

            // Show the results of the reverse geocoding in a toast.
            resultAddress = address.toString();
            Toast.makeText(curActivity, resultAddress, Toast.LENGTH_LONG).show();
        }
    }

}

When i long click on mapview this error will come com.esri.core.io.EsriServiceException: Cannot perform query. Invalid query parameters and toast will show all parameters null except city.. I think i'm doing some mess with Tiled service on Dynamic service.. Please help what i did wrong..

Thanks in Advance