Bag nHibernate with key null

Hello,

Does anyone know if it's possible to use nHibernate's Bag with Key Null?

In a table some records have several related records, one for many, but not always, sometimes a regret will not have any.

I made the mapping by a bag but the foreign key error because the field in the related table will sometimes be null.

Is there any way to bring a registry collection this way that I need.

Here is the code for classes and mappings:

Class that has the parent records:

public class Fatura
{
    public virtual long Id { get; set; }
    public virtual DateTime DtEmissao { get; set; }
    public virtual decimal VlrTotal { get; set; }
    public virtual Convenio Convenio { get; set; }
    public virtual IList<Receber> Receber { get; set; }

    public Fatura()
    {
        Receber = new List<Receber>();
    }

}

Class that has the child records:

public class Receber
{
    public virtual long Id { get; set; }
    public virtual Fatura Fatura { get; set; }
    public virtual DateTime Pagamento { get; set; }
    public virtual decimal Valor { get; set; }
    public virtual decimal Juros { get; set; }
    public virtual decimal Multa { get; set; }
    public virtual decimal OutrosAcrescimos { get; set; }
    public virtual decimal Desconto { get; set; }
}

Parent class mapping:

public class FaturaMapping : ClassMapping<Fatura>
{
    public FaturaMapping()
    {
        Table("FATURA");

        Id(x => x.Id, x =>
        {
            x.Column("ID_FATURA");
        });

        Property(x => x.DtEmissao, x =>
        {
            x.Column("DTEMISSAO");
        });

        Property(x => x.VlrTotal, x =>
        {
            x.Column("VLRTOTAL");
        });

        ManyToOne(x => x.Convenio, x =>
        {
            x.Column("FK_CONVENIO");
            x.NotNullable(true);
        });

        Bag(x => x.Receber, x =>
        {
            x.Key(y =>
            {
                y.Column("FK_FATURA");
                y.NotNullable(false);
            });
        }, x => x.OneToMany());

    }
}