AssertionFailure: “пустой идентификатор” - FluentNH + SQLServerCE

Код перестал работать в

session.Save(employee);

using FluentNHibernate.Cfg;
using FluentNHibernate.Cfg.Db;
using FluentNHibernate.Mapping;
using NHibernate;
using NHibernate.Cfg;
using NHibernate.Tool.hbm2ddl;

namespace FNHTest
{
    public class Employee
    {
        public virtual int Id
        {
            get;
            private set;
        }

        public virtual string Name
        {
            get;
            set;
        }

        public virtual string Surname
        {
            get;
            set;
        }
    }

    public class EmployeeMap : ClassMap
    {
        public EmployeeMap()
        {
            Id(e => e.Id);
            Map(e => e.Name);
            Map(e => e.Surname);
        }
    }

    public class DB
    {
        private static ISessionFactory mySessionFactory = null;

        private static ISessionFactory SessionFactory
        {
            get
            {
                if (mySessionFactory == null)
                {
                    mySessionFactory = Fluently.Configure()
                        .Database(MsSqlCeConfiguration.Standard
                                    .ConnectionString("Data Source=MyDB.sdf"))
                        .Mappings(m => m.FluentMappings.AddFromAssemblyOf())
                        .ExposeConfiguration(BuildSchema)
                        .BuildSessionFactory();
                }
                return mySessionFactory;
            }
        }

        private static void BuildSchema(Configuration configuration)
        {
            SchemaExport schemaExport = new SchemaExport(configuration);
            schemaExport.Execute(false, true, false);
        }

        public static ISession OpenSession()
        {
            return SessionFactory.OpenSession();
        }
    }

    public class Program
    {
        public static void Main(string[] args)
        {
            var employee = new Employee
            {
                Name = "John",
                Surname = "Smith"
            };

            using (ISession session = DB.OpenSession())
            {
                session.Save(employee);
            }
        }
    }
}