WebException.状态未序列化

当我运行下面的测试(NUnit和VS2008SP1)时,我得到以下错误

预期:ConnectFailure
但是:未知错误

有人能解释我做错了什么吗?

[Test]
public void SerializationWebExceptionTest()
{
    using (var stream = new MemoryStream())
    {
        const WebExceptionStatus Expected = WebExceptionStatus.ConnectFailure;
        var formatter = new BinaryFormatter();
        var initialException = new WebException("Test", null, Expected, null);

        formatter.Serialize(stream, initialException);
        stream.Seek(0, SeekOrigin.Begin);
        var result = (WebException)formatter.Deserialize(stream);

        var actual = result.Status;

        Assert.That(actual, Is.EqualTo(Expected));
    }
}