sharp arch validator and entity with typed id

Song.cs

    [HasUniqueDomainSignatureWithGuidId]
    public class Song : EntityWithTypedId<Guid>
    {
        public Song()
        {           
        }

        [DomainSignature]
        public virtual string SongTitle { get; set; }

        [DomainSignature]
        public virtual Band Performer { get; set; }
    }

spec.cs

        public class when_validating_a_duplicate_entity_that_has_a_null_value_domain_signature_property : specification_for_has_unique_domain_signature_validator
        {
            static Song duplicateSong;
            static bool result;

            private Establish context = () =>
            {
                var song = new Song() { SongTitle = "Souvenirs" };
                NHibernateSession.Current.Save(song);
                NHibernateSession.Current.Flush();
                duplicateSong = new Song() { SongTitle = "Souvenirs" };
            };

            Because of = () => result = duplicateSong.IsValid();

            It should_say_the_entity_is_invalid = () => result.ShouldBeFalse();
        }