nullを許容しない型はnull許容型で取得しようね <http://msdn.microsoft.com/ja-jp/library/vstudio/2cf62fcy.aspx>

DictionaryWithNullValueTest.cs

using NUnit.Framework;
using System.Collections.Generic;

[TestFixture]
public class DictionaryWithNullValueTest
{
	[Test]
	public void DictionaryCanGetNullableInteger()
	{
		Dictionary<string, object> dict = (Dictionary<string, object>) MiniJSON.Json.Deserialize("{ \"hoge\": null }");
		Assert.True(dict.ContainsKey("hoge"));
		int? hoge = (int?) dict["hoge"];
		Assert.IsNull(hoge);
	}
}