Error 400 Bad Request when accessing google APIs

Question

I am trying to access google Knowledge graph API except am encountering the error com.google.api.client.http.HttpResponseException: 400 Bad Request "message": "API key not valid. Please pass a valid API key.", but I do not get it, my API key is the right one I obtained based on the instructions supplied here

  HttpTransport httpTransport = new NetHttpTransport();
        HttpRequestFactory requestFactory = httpTransport.createRequestFactory();
        JSONParser parser = new JSONParser();
        GenericUrl url = new GenericUrl("https://kgsearch.googleapis.com/v1/entities:search");
        url.put("query", "Taylor Swift");
        url.put("limit", "10");
        url.put("indent", "true");
        url.put("key",R.string.API_KEY);
        HttpRequest request = requestFactory.buildGetRequest(url);
        HttpResponse httpResponse = request.execute();