How to properly unlockCanvas?

Question

Im using

public class MyWallpaperService extends WallpaperService

public Engine onCreateEngine() {
      return new MyWallpaperEngine();
    }

in MyWallpaperEngine im doing this after touch

private class MyWallpaperEngine extends Engine



public void onTouchEvent(MotionEvent event) {
              SurfaceHolder holder = getSurfaceHolder();
              Canvas canvas = null;
              try {
                  canvas = holder.lockCanvas();
                  drawCircles(canvas, circles);
              }
                finally {
                if (canvas != null)
                  holder.unlockCanvasAndPost(canvas);  //this is 86 line
              }
              super.onTouchEvent(event);
          }

but it throws a Exceptions:

11-16 20:36:14.556: E/Surface(7304): queueBuffer: error queuing buffer to SurfaceTexture, -19
11-16 20:36:14.556: E/Surface(7304): queueBuffer (handle=0xb7bc2a30) failed (No such device)
11-16 20:36:14.556: E/AndroidRuntime(7304): FATAL EXCEPTION: main
11-16 20:36:14.556: E/AndroidRuntime(7304): java.lang.IllegalArgumentException
11-16 20:36:14.556: E/AndroidRuntime(7304):     at android.view.Surface.nativeUnlockCanvasAndPost(Native Method)
11-16 20:36:14.556: E/AndroidRuntime(7304):     at android.view.Surface.unlockCanvasAndPost(Surface.java:255)
11-16 20:36:14.556: E/AndroidRuntime(7304):     at com.android.internal.view.BaseSurfaceHolder.unlockCanvasAndPost(BaseSurfaceHolder.java:215)
11-16 20:36:14.556: E/AndroidRuntime(7304):     at com.example.wallparer.MyWallpaperService$MyWallpaperEngine.onTouchEvent(MyWallpaperService.java:86)

Have no idea why? ...