NHibernate - LAZY LOADING PROBLEM -Initializing[]-Could not initialize proxy - no Session."}

Hi I use Fluent NHibernate and I am little confusing with Lazy Loading.

1. I queried object in database
2. modified object properties
3. update database with this object

Here is code:

public class Credentials
{
    public virtual int Id { get; set; }
    public virtual string Nick { get; set; }
    public virtual string Password { get; set; }
}

public class CredentialsMap : ClassMap<Credentials>
{
    public CredentialsMap()
    {
        Id(x => x.Id);
        Map(x => x.Nick).Column("NICK");
        Map(x => x.Password).Column("PASSWORD");
        Table("R_CREDENTAILS");
    }
}

public class Status
{
    public virtual int Id { get; set; }
    public virtual string Msg { get; set; }
    public virtual DateTime AddTime { get; set; }
}

public class StatusMap : ClassMap<Status>
{
    public StatusMap()
    {
        Id(x => x.Id);
        Map(x => x.Msg).Column("MESSAGE");
        Map(x => x.AddTime).Column("ADD_TIME");
        Table("R_STATUS");
    }
}

public class Account
{
    public virtual int Id { get; set; }
    public virtual string SelfNick { get; set; }
    public virtual Credentials Credentials { get; set; }
    public virtual Status Status { get; set; }
}

public class AccountMap : ClassMap<Account>
{
    public AccountMap()
    {
        Id(x => x.Id);
        Map(x => x.SelfNick).Column("SELF_NICK");
        References(x => x.Credentials)
            .Column("CREDENTIALS_ID")
            .ForeignKey();
        References(x => x.Status)
            .Column("STATUS_ID")
            .ForeignKey();
        Table("R_ACCOUNTS");
    }
}

NHibernate configuration class:

public class NHiberanteHelper
{
    private static ISessionFactory _sessionFactory;

    private static ISessionFactory SessionFactory
    {
        get
        {
            if (_sessionFactory == null)
                InitializeSessionFactory();