NHibernate use Firebird own generator

Question

Having own generator in Firebird DB called GEN_PATIENT_ID I would like to leave the generation of ID on the DB side instead of NHibernate and FluentNHibernate. Following this link I am overriding the default Fluent behavior like this:

public class PatientOverride : IAutoMappingOverride<Patient>
    {
        public void Override(AutoMapping<Patient> mapping)
        {
            mapping.Cache.ReadOnly().Region("LongTermReadWrite");
            mapping.Id(x => x.Id).GeneratedBy.Sequence("GEN_PATIENT_ID");
            mapping.HasOne(patient => patient.Tag).Not.LazyLoad().Cascade.All();
        }
    }

However I still get an error because NHibernate is trying to generate its own:

{"Dynamic SQL Error\r\nSQL error code = -204\r\nTable unknown\r\nHIBERNATE_UNIQUE_KEY\r\nAt line 1, column 42"}

Doublechecked if I am importing the overrides and it should be OK:

mappings.UseOverridesFromAssemblyOf<AutoPersistenceModelGenerator>();

the save is pretty much standard:

var patient = patientDtoMapper.MapFrom(patientDto);

using (ITransaction t = NHibernateSession.Current.BeginTransaction())
{
    patientRepository.Save(patient);                
    t.Commit();
}

Any ideas?