The following example leads to Xtensive.Orm.QueryTranslationException

public class Program
{
    public static void Main(string[] args)
    {
        var dc = new DomainConfiguration("sqlserver", "Data Source=.; Initial Catalog=DO40-Tests; Integrated Security=True;");

        dc.Types.Register(typeof(TestEntity));
        dc.Types.Register(typeof(TestEntity2));

        dc.UpgradeMode = DomainUpgradeMode.Recreate;

        using (var d = Domain.Build(dc))
        {
            using (var s = d.OpenSession())
            using (s.Activate())
            using (s.OpenTransaction())
            {
                var test = new TestEntity2(s) { Str = "test" };
                new TestEntity(s) { TestEntity2 = test, Str = "test" };

                // This query is successfull
                var items1 = Query.All<TestEntity>()
                                 .Select(it => new
                                  {
                                      Test = it.TestEntity2 != null
                                                 ? string.Empty
                                                 : it.TestEntity2.Str
                                  })
                                 .Select(it => it.Test == null || it.Test.Length < 10
                                                   ? it.Test
                                                   : it.Test.Substring(0, 10))
                                 .ToList();

                // This query gets exception
                var items2 = Query.All<TestEntity>()
                                 .Select(it => new
                                  {
                                      Test = it.TestEntity2 != null
                                                 ? (string)null
                                                 : it.TestEntity2.Str
                                  })
                                 .Select(it => it.Test == null || it.Test.Length < 10
                                                   ? it.Test
                                                   : it.Test.Substring(0, 10))
                                 .ToList();
            }
        }  
    }

    [HierarchyRoot]
    public class TestEntity : Entity
    {
        /// <summary>Initializes a new instance of this class.</summary>
        /// <param name="session">The session.</param>
        public TestEntity(Session session) : base(session)
        {
        }

        [Key]
        [Field(Nullable = false)]
        public int Id { get; private set; }

        /// <summary>
        /// TestEntity2
        /// </summary>
        [Field]
        public TestEntity2 TestEntity2 { get; set; }

        /// <summary>
        /// Str
        /// </summary>
        [Field]
        public string Str { get; set; }
    }

    [HierarchyRoot]
    public class TestEntity2 : Entity
    {
        /// <summary>Initializes a new instance of this class.</summary>
        /// <param name="session">The session.</param>
        public TestEntity2(Session session) : base(session)
        {
        }

        [Key]
        [Field(Nullable = false)]
        public int Id { get; private set; }

        /// <summary>
        /// Str
        /// </summary>
        [Field]
        public string Str { get; set; }
    }
}