Fluent NHibernate: Insert NULL into FK column

I have to two tables in my database: Patients and Addresses. They are in one-to-one relation throught Address and CorrespodencyAddress fields in Patients.

Here is the code for Patients:

public class Patients
{
    public virtual int Id { get; protected set; }
    public virtual int? IndividualId { get; set; }
    public virtual string First_Name { get; set; }
    public virtual string Last_Name { get; set; }
    public virtual string Pesel { get; set; }
    public virtual string Gender { get; set; }
    public virtual string Height { get; set; }       //wzrost
    public virtual string Comments { get; set; }     //uwagi

    public virtual Adresses Address { get; set; }
    public virtual Adresses CorrespondencyAddress { get; set; }
    public virtual StudyPayer DefaultPayer { get; set; }

    public virtual DateTime? BirthDate { get; set; }
    public virtual string PhoneNumber { get; set; }

    public virtual DateTime? RegistrationDate { get; set; } 
    public virtual IList<Studies> Studies { get; set; } 

    public Patients()
    {
        Studies = new List<Studies>();
    }

    public virtual void AddAddress(Adresses adresses)
    {
        adresses.Patient = this;
        Address = adresses;
    }

    public virtual void AddStudy(Studies studies)
    {
        studies.Patient = this;
        Studies.Add(studies);
    }
}


public class PatientsMap :ClassMap<Patients>
{
    PatientsMap()
    {
        Id(x => x.Id).GeneratedBy.Identity();
        Map(x => x.IndividualId);
        Map(x => x.First_Name);
        Map(x => x.Last_Name);
        Map(x => x.Pesel);
        Map(x => x.Gender);

        if (ConfigurationMap.DbType == DbType.PostgreSql)
        {
            Map(x => x.RegistrationDate).Nullable();
            Map(x => x.BirthDate).Nullable();
        }

        if (ConfigurationMap.DbType == DbType.MsSql)
        {
            Map(x => x.RegistrationDate).CustomSqlType("datetime2").Nullable();
            Map(x => x.BirthDate).CustomSqlType("datetime2").Nullable();    
        }

        Map(x => x.PhoneNumber);
        Map(x => x.Height);
        Map(x => x.Comments);

        References(x => x.Address).Cascade.All();
        References(x => x.CorrespondencyAddress).Cascade.All();

        References(x => x.DefaultPayer);
        HasMany(x => x.Studies).Cascade.All().KeyColumn("patient_id");
    }
}

Here is code for Addresses

public class Adresses
{
    public virtual int Id { get; protected set; }
    public virtual string Street { get; set; }
    public virtual string HomeNumber { get; set; }
    public virtual string PostalCode { get; set; }
    public virtual string City { get; set; }
    public virtual string Country { get; set; }

    public virtual Patients Patient { get; set; }

    public virtual void AddPatient(Patients patient)
    {
        patient.Address = this;
        Patient = patient;
    }

}    


public class AdressesMap : ClassMap<Adresses>
{
    AdressesMap()
    {
        Id(x => x.Id).GeneratedBy.Identity();
        Map(x => x.HomeNumber);
        Map(x => x.Street);
        Map(x => x.PostalCode);
        Map(x => x.City);
        Map(x => x.Country);

        References(x => x.Patient).Cascade.All();
    }
}

As we all know sometimes CorrespodencyAddress is the same as Address, then I'd like to insert NULL into CorrespodencyAddress and it will be meaning it is the same address.

How can I achieve that?

If I do like below that I get "Nullable object must have a value" exception in runtime:

Patients p = new Patients();
p.CorrespondencyAddress = null;