Commented Unassigned: Tweet favourite [2096]

I am trying to mark a tweet as a favourite, I have the authorization token and am able to view profile details. When I search for tweets I get back list but I am unable to mark any as a favourite, I receive and exception and a message "Value cannot be null. Parameter name: s"

System.ArgumentNullException was unhandled by user code
HResult=-2147467261
Message=Value cannot be null.
Parameter name: s
Source=mscorlib
ParamName=s
StackTrace:
at System.IO.StringReader..ctor(String s)
at Newtonsoft.Json.Linq.JObject.Parse(String json)
at TweetinviLogic.Wrapper.JObjectStaticWrapper.GetJobjectFromJson(String json)
at TweetinviCredentials.TwitterAccessor.ExecutePOSTQuery(String query)
at TweetinviCredentials.TwitterAccessor.TryExecutePOSTQuery(String query, JsonConverter[] converters)
at TweetinviControllers.Tweet.TweetQueryExecutor.FavouriteTweet(ITweetDTO tweet)
at TweetinviControllers.Tweet.TweetController.FavoriteTweet(ITweetDTO tweetDTO)
at TweetinviLogic.Tweet.Favourite()
at TwitterBot.UnitTest1.search() in c:\Users\andrew\Documents\Visual Studio 2013\Projects\TwitterBot\TwitterBot\UnitTest1.cs:line 89
InnerException:
Comments: ** Comment from web user: digitalgym **

It turns out that when I search tweets by hash tags like

var hash = System.Uri.EscapeDataString("#melbourne AND #realestate");
var tweets = Search.SearchTweets(hash);

These tweet can not be favourite but when I search tweets by

var tweets = Search.SearchTweets("real estate");

There is no problem, is the tweet different somehow?