How do you create a map without generics that can ignore destination properties? Automapper v1.1

AutomapperMapSameType.cs

    public class Object1
    {
        public int Id { get; set; }
        public string FirstField { get; set; }
        public string SecondField { get; set; }
    }

        public class WhenMappingTwoItemsOfTheSameType : Spec
        {

            protected Object1 original;
            protected Object1 update;

            public override void Context()
            {
                 original = new Object1
                {
                    Id = 123,
                    FirstField = "a",
                    SecondField = "1"
                };

                update = new Object1
                {
                    Id = 0,
                    FirstField = "b",
                    SecondField = "2"
                };

                //Mapper.CreateMap(original.GetType(), original.GetType()).ForMember("Id",opt => opt.Ignore()); //doesn't compile
            }

            public override void Because()
            {
                //original = Mapper.Map(update, original);
                Mapper.DynamicMap(update, original);
            }

            [Fact]
            public void ShouldNotReplaceId()
            {
                original.Id.ShouldEqual(123); //fails
            }

            [Fact]
            public void ShouldReplaceFirstField()
            {
                original.FirstField.ShouldEqual(update.FirstField);
            }
        }

The solution? Write an extension or abstraction on top of the Dynamic Map that will catch the fields you want to keep and set them back. Not pretty...but it works. :D