Class is not supported for deserialization of an array

I get an error:

System.InvalidOperationException: Type 'VK.Response' is not supported for deserialization of an array.

JSON that i get looks like:

{"response":[{"id":269058571,"first_name":"Name","last_name":"LastName","photo_50":"http://ift.tt/1JxM05L"}]}

My method is:

 private void getFriendInfo()
    {
        string method = "users.get";
        string param = "user_ids=269058571&fields=photo_50";
        string url = "http://ift.tt/1huYTUH" + method + "?" + param + "&v=5.31&access_token=" + key + "";

        WebClient client = new WebClient();

        string json = client.DownloadString(url);
        JavaScriptSerializer json_serializer = new JavaScriptSerializer();
        RootObject response = (RootObject)json_serializer.Deserialize(json, typeof(RootObject));
        for (int counter = 0; counter < response.response.items.Count; counter++)
        {
            pictureBox1.Load(response.response.items[counter].photo_50);
        }
    }

Classes:

      class Response
        {
            public int count { get; set; }
            public List<Item> items { get; set; }
        }

       class RootObject
       {
            public Response response { get; set; }
       }
       class Item
       {
            public string first_name { get; set; }
            public string last_name { get; set; }
            public string domain { get; set; }
            public string photo_50 { get; set; }
       }