Android error parsing post response org.json.JSONException: End of input at character 0 of

I'm writing an Android app that can parse JSON returned from PHP webpages, and I keep getting "org.json.JSONException: End of input at character 0" in Logcat. Here's the code:

public class HttpConnect {
    HttpClient httpClient = new DefaultHttpClient();
    final String url = "http://foo/bar/handler.php";
    HttpPost httpPost = new HttpPost(url);
    HttpParams httpParams = new BasicHttpParams();
    HttpResponse httpResponse;
    List<NameValuePair> params = new ArrayList<NameValuePair>();
    JSONObject resultJson;
    boolean status;

    public boolean sendJson(String value) {
        try {
        params.add(new BasicNameValuePair("jsonString", URLEncoder.encode(
                value, "utf-8")));
        httpPost.setEntity(new UrlEncodedFormEntity(params, HTTP.UTF_8));
        httpResponse = httpClient.execute(httpPost);
        if (httpResponse.getStatusLine().getStatusCode() == 200) {
            String result = EntityUtils.toString(httpResponse.getEntity());
            resultJson = new JSONObject(result);
        }
        if (resultJson.getString("success") == "true")
            status = true;
        else
            status = false;
        } catch (Exception e) {
        e.printStackTrace();
        }
        return status;
    }
    //...
}

Server side:

$arr = array('success' => 'true', 'status' => 'RegSuccess', 'userid' => $userid, 'authkey' => $authKey);
$jsonString = json_encode($arr);
echo $jsonString;

Postman gets the response, which is correct:

{
    "success": "true",
    "status": "RegSuccess",
    "userid": "2",
    "authkey": "OvOWwCjIXoYtbjsIdTVI"
}

But the android app pops error:

08-07 16:44:34.044: W/System.err(13880): org.json.JSONException: End of input at character 0 of 08-07 16:44:34.049: W/System.err(13880): at org.json.JSONTokener.syntaxError(JSONTokener.java:450) 08-07 16:44:34.049: W/System.err(13880): at org.json.JSONTokener.nextValue(JSONTokener.java:97) 08-07 16:44:34.049: W/System.err(13880): at org.json.JSONObject.(JSONObject.java:155) 08-07 16:44:34.049: W/System.err(13880): at org.json.JSONObject.(JSONObject.java:172) 08-07 16:44:34.049: W/System.err(13880): at com.lafickens.rescued.HttpConnect.sendJson(HttpConnect.java:49) 08-07 16:44:34.049: W/System.err(13880): at com.lafickens.rescued.LoginActivity$UserRegisterTask.sendCredentials(LoginActivity.java:369) 08-07 16:44:34.049: W/System.err(13880): at com.lafickens.rescued.LoginActivity$UserRegisterTask.doInBackground(LoginActivity.java:325) 08-07 16:44:34.049: W/System.err(13880): at com.lafickens.rescued.LoginActivity$UserRegisterTask.doInBackground(LoginActivity.java:1) 08-07 16:44:34.049: W/System.err(13880): at android.os.AsyncTask$2.call(AsyncTask.java:288) 08-07 16:44:34.054: W/System.err(13880): at java.util.concurrent.FutureTask.run(FutureTask.java:237) 08-07 16:44:34.054: W/System.err(13880): at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:231) 08-07 16:44:34.054: W/System.err(13880): at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1112) 08-07 16:44:34.054: W/System.err(13880): at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:587) 08-07 16:44:34.054: W/System.err(13880): at java.lang.Thread.run(Thread.java:841)

Debugging shows string result is empty, and contentLength of httpResponse is 0. The same java code works perfectly fine for another web service, but not for mine. Puzzling :s

Any help is appreciated:)