AsyncTask 不能在 4.0.4 上运行

我创建了一个类，这个类连接互联网，然后发送一个请求到php script。我创建的就像 AsyncTask一样，不在主线程中，为了能在4.0.4中执行。但是当我测试时，不能运行。程序可以在2.2上执行。问题出在哪里呢？

class download extends AsyncTask {
    protected String doInBackground(String s1, String s2) {
        String result = "";
        //http post
        ArrayList nameValuePairs = new ArrayList();
        nameValuePairs.add(new BasicNameValuePair("Vreme",s1));
        nameValuePairs.add(new BasicNameValuePair("Datum",s2));
        InputStream is=null;
        try{
                String adresa="http://senzori.open.telekom.rs/script.php";
                HttpPost httppost = new HttpPost(adresa);
                httppost.setEntity(new UrlEncodedFormEntity(nameValuePairs));
                HttpClient httpclient = new DefaultHttpClient();
                HttpResponse response = httpclient.execute(httppost);
                HttpEntity entity = response.getEntity();
                is = entity.getContent();
        }catch(Exception e){
                Log.e("log_tag", "Error in http connection "+e.toString());
        }
        //convert response to string
        try{
                BufferedReader reader = new BufferedReader(new InputStreamReader(is,"iso-8859-1"),8);
                StringBuilder sb = new StringBuilder();
                String line = null;
                while ((line = reader.readLine()) != null) {
                        sb.append(line + "\n");
                }
                is.close();
                result=sb.toString();
        }catch(Exception e){
                Log.e("log_tag", "Error converting result "+e.toString());
        }
        return result;
    }
    @Override
    protected String doInBackground(String... params) {
        // TODO Auto-generated method stub
        return null;
    }
}

doInBackground(String... params) 怎么不是用的这个可变参数的方法，覆写应该是覆写这个方法啊

 @Override
    protected String doInBackground(String... params) {
        // TODO Auto-generated method stub
        return null;
    }