RuntimeException while httpclient.execute in BackgroundService

Question

            HttpClient httpClient = new DefaultHttpClient();
        HttpPost httppost = new HttpPost(urlString);

        HttpResponse response = null;
        MultipartEntity mpEntity = new MultipartEntity();

        StringBody sb;
        String result = null;

        try {
            sb = new StringBody(json.toString());
            mpEntity.addPart("json", sb);

            httppost.setEntity(mpEntity);
            response = httpClient.execute(httppost);

            HttpEntity entity = response.getEntity();
            if (entity != null) {
                InputStream instream = entity.getContent();
                result = RestClient.convertStreamToString(instream);
                Log.i("Read from Server", result);

            }

        } catch (UnsupportedEncodingException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (ClientProtocolException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

        if(result == "true"){
            this.stopSelf();
        } else {

        }

I get an RuntimeException in my Background Service on:

response = httpClient.execute(httppost);

I don't know why :( I do the same not in a Service and everything's ok, but in Service it throws the exception.