New Post: json::value v[L"responseData"] - error (there is no operator "[]")

I never before use C++ REST and json and found an example of use in this blog - [http://mariusbancila.ro/blog/2013/08/02/cpp-rest-sdk-in-visual-studio-2013 /]
(C++ REST SDK in Visual Studio 2013)

there is code:

#include <cpprest/http_client.h>
#include <cpprest/filestream.h>
#include <cpprest/json.h>
 
#include <iostream>
 
using namespace utility;
using namespace web;
using namespace web::http;
using namespace web::http::client;
using namespace concurrency::streams;
 
using namespace std;
 
void print_search_results(json::value const & value)
{
   if(!value.is_null())
   {
      auto response = value[L"responseData"];
      auto results = response[L"results"];
      for(auto const & p : results)
      {
         auto o = p.second;
         auto url = o[L"url"];
         auto title = o[L"titleNoFormatting"];
 
         wcout << title.as_string() << endl << url.as_string() << endl << endl;
      }
   }
}
 
void search_and_print(wstring const & searchTerm, int resultsCount)
{
   http_client client(U("https://ajax.googleapis.com/ajax/services/search/web"));
 
   // build the query parameters
   auto query =  uri_builder()
      .append_query(L"q", searchTerm)
      .append_query(L"v", L"1.0")
      .append_query(L"rsz", resultsCount)
      .to_string();
 
   client
      // send the HTTP GET request asynchronous
      .request(methods::GET, query)
      // continue when the response is available
      .then([](http_response response) -> pplx::task<json::value>
      {
         // if the status is OK extract the body of the response into a JSON value
         // works only when the content type is application\json
         if(response.status_code() == status_codes::OK)
         {
            return response.extract_json();
         }
 
         // return an empty JSON value
         return pplx::task_from_result(json::value());
      })
      // continue when the JSON value is available
      .then([](pplx::task<json::value> previousTask)
      {
         // get the JSON value from the task and display content from it
         try
         {
            json::value const & v = previousTask.get();
            print_search_results(v);
         }
         catch (http_exception const & e)
         {
            wcout << e.what() << endl;
         }
      })
      .wait();
}

int main(int argc, char *args[])
{
   search_and_print(L"marius bancila", 5);
 
   return 0;
}

but I can't build it!
error on

auto response = value[L"responseData"];

what's wrong??