Linqメソッドで遊ぶ 例外

IEnumerable<>.Union

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace Linqメソッドで遊ぶ
{
	class Program
	{
		static void Main(string[] args)
		{
			var hoge1List = new List<inheritHoge1>();
			hoge1List.Add(new inheritHoge1() { a = "hoge1" });

			var hoge2List = new List<inheritHoge2>();
			var hoge = new inheritHoge2() { a = "hoge3" };
			hoge2List.Add(hoge);
			hoge2List.Add(new inheritHoge2() { a = "hoge2" });
			hoge2List.Add(new inheritHoge2() { a = "hoge1" });
			hoge2List.Add(hoge);

			var union = hoge1List.Select(x => (Hoge)x).Union(hoge2List);

			foreach (var item in union)
			{
				Console.WriteLine(item.a);
			}
			Console.Read();
		}
	}

	class Hoge
	{
		public string a = string.Empty;
	}

	class inheritHoge1 : Hoge
	{
	}

	class inheritHoge2 : Hoge
	{
	}
}