Mono variance validity issue?

gistfile1.cs

// ContravariantIssue.cs(3,26): error CS1961: The contravariant type parameter `TSomething' must be 
// covariantly valid on `ContravariantIssue.In.Mono.Example.IInterface<TSomething>.Something'

namespace ContravariantIssue.In.Mono.Example
{
	public interface IInterface<in TSomething>
	{
		TSomething Something { set; }
	}

	public interface TSomething
	{
		string FieldOne { get; set; }
		string FieldTwo { get; set; }
	}

	public class Something : TSomething
	{
		public string FieldOne { get; set; }
		public string FieldTwo { get; set; }
	}

	public class ContraExample : IInterface<Something>
	{
		public Something Something { private get; set; } 
	}

	class Program
	{
		static void Main(string[] args)
		{

		}
	}
}