JSON parser failing to Convert my Strings

I am working on an Android App that Collects certain information from a PHP MysQl Server, but I am facing a problem with JSON Parser, sometimes it works sometimes it does not work, it gives me these Errors:

    E/JSON Parser﹕ Error parsing data org.json.JSONException: Value Failed of type java.lang.String cannot be converted to JSONObject
    Caused by: java.lang.NullPointerException

And the Code thats producing the Errors:

try {
                // Building Parameters
                List<NameValuePair> params = new ArrayList<NameValuePair>();
                params.add(new BasicNameValuePair("username", username));
                params.add(new BasicNameValuePair("password", password));
                params.add(new BasicNameValuePair("fname", fname));
                params.add(new BasicNameValuePair("lname", lname));
                params.add(new BasicNameValuePair("email", email));
                params.add(new BasicNameValuePair("city", city));
                params.add(new BasicNameValuePair("nrc", nrc));
                params.add(new BasicNameValuePair("dob", dob));
                params.add(new BasicNameValuePair("cell", cell));




                Log.d("username", username);
                Log.d("password", password);
                Log.d("fname", fname);
                Log.d("lname",lname );
                Log.d("email",email );
                Log.d("city",city );
                Log.d("nrc",nrc );
                Log.d("dob",dob );
                Log.d("cell",cell );



                Log.d("request!", "starting");

                //Posting user data to script
                JSONObject json = jsonParser.makeHttpRequest(
                        REGISTER_URL, "POST", params);

                // full json response
                Log.d("Registering attempt", json.toString());


                // json success element
                success = json.getInt(TAG_SUCCESS);
                if (success == 1) {
                    Log.d("User Profile Created!", json.toString());

The NullException is being caused by the Line:

    Log.d("Registering attempt", json.toString());