Creating JProperty from Dictionary with LINQ in JSON.NET

I need to get JSON

{
   "question":"q1",
   "answ开发者_开发技巧ers": {
       1:"ans1",
       2:"ans2",
       3:"ans3"
    }
    "corr":[1,2]
 }

with this expression contains LINQ

JObject jsonContent =
            new JObject(
                new JProperty("question", _question),
                new JProperty("answers",
                    new JObject(
                        from ans in _answers
                        select new JProperty (ans.Key.ToString(),ans.Value))),
                new JProperty("corr",
                    new JArray(
                        from ans in _correctAnswers
                        select ans)));

where

string _question;
List<int> _correctAnswers;
Dictionary<int, string> _answers;

I have a problem with converting Dictionary into JProperty

System.ArgumentNullException: Value cannot be null.
Parameter name: source

UPD: The all of values are set. There is no null answer

UPD2: Sorry. All works fine. The problem was in db-access layer

Looks like there might be a answer in the Dictionary that is null

try: select new JProperty (ans.Key.ToString(),ans.Value ?? string.Empty)

To convert a dictionary to JProperty I serialize and parse it again:

var dict = new Dictionary<string, string>();
// add items

var prop = new JProperty("yourprop", JObject.Parse(JsonConvert.SerializeObject(dict.Values)));

I Hope it could be of any help.