Fluent NHibernate mapping to a non-public field

gistfile1.cs

    public class TestClass
    {
        private decimal myField;
        public virtual int Id { get; set; }
        public virtual string Name { get; set; }
        public virtual void SetField(decimal newOne)
        {
            myField = newOne;
        }

        public TestClass()
        {
            myField = 50m;
        }

        public TestClass(Decimal x)
        {
            myField = x;
        }
    }

    public class MyMapping: ClassMap<TestClass>
    {
        public MyMapping()
        {
            Map(Reveal.Member<TestClass>("myField"));
            Id(x => x.Id);
            Map(x => x.Name);
        }
    }
    
    public static void Main(params string[] args)
    {
      var config = Fluently.Configure()
                  .Database(
                  MsSqlConfiguration.MsSql2008.ShowSql().ConnectionString(
                  c => c.FromConnectionStringWithKey("db")))
                  .Mappings(x => x.FluentMappings.AddFromAssembly(Assembly.GetExecutingAssembly())).BuildConfiguration();

      new SchemaExport(config).Create(true, true);

      using(var factory = config.BuildSessionFactory())
      {
          using(var session = factory.OpenSession())
          {
              session.SaveOrUpdate(new TestClass(5.0m) { Name = "Some name" });
              var fetched = session.Query<TestClass>().SingleOrDefault(x => x.Id == 1);
              fetched.SetField(12m);
              session.SaveOrUpdate(fetched);
              session.Flush();
          }
      }
    }