One to many from a derived class to other entity generates weird mapping

OneToManyFromDerivedFluentMapping

class Product
        {
            public int ProductId { get; set; }
        }

        class SpecialProduct : Product
        {
            public ICollection<Option> Options { get; set;}
        }

        class Option
        {
            public int OptionId { get; set; }
            public SpecialProduct Back { get; set; }
        }

        class ProductMap : ClassMap<Product>
        {
            public ProductMap()
            {
                Id(x => x.ProductId);
            }
        }

        class SpecialProductMap : SubclassMap<SpecialProduct>
        {
            public SpecialProductMap()
            {
                Extends<Product>();
                HasMany(x => x.Options).Cascade.AllDeleteOrphan();
            }
        }

        class OptionMap : ClassMap<Option>
        {
            public OptionMap()
            {
                Id(x => x.OptionId);
            }
        }