I have a class like:

	public class TagRecord
	{
		public Guid Id { get; set; }
		....
		public ExpandoObject ExtraData { get; set; }
	}

When I trying to insert it in MongoDb without setting ExtraData property (so leaving it to null) I get a NullReferenceException exception.
If I simply change the type to "Object" all is working fine, also when I actually save it with an ExpandoObject instance.