C# – Fluent NHibernate : How to i use Int64 as ID

I'm quite new to the whole concept of ORM, NHibernate and FluentNH, and i'm trying to accomplish something that seems so simple…

I'm trying to retrieve an object that has a field defined as Int64 in it's class. This field would be the ID, as defined in the Map file.

When trying to retrieve a record from the DB, NHibernate returns the following error message : "Provided id of the wrong type. Expected: System.Int32, got System.Int64"

I have a very simple object :

public class Holiday
{
    public virtual Int64 HolidayID { get; set; }
    public virtual string Name { get; set; }
    public virtual int Day { get; set; }
    public virtual int Month { get; set; }
    public virtual bool IsActive { get; set; }
    public virtual HolidayGroup Group { get; set; }

    public Holiday() { }

}

the Mapping file (for FluentNH) is the following :

public class HolidayMap : ClassMap<Holiday>
{
    public HolidayMap()
    {

        Id(x => x.HolidayID);
        Map(x => x.Name);
        Map(x => x.Day);
        Map(x => x.Month);
        Map(x => x.IsActive);
        HasOne(x => x.Group);
    }
}

the table structure (generated by NH) is the following:

CREATE TABLE [dbo].[Holiday](
 [HolidayID] [bigint] IDENTITY(1,1) NOT NULL,
 [Name] [nvarchar](255) NULL,
 [Day] [int] NULL,
 [Month] [int] NULL,
 [IsActive] [bit] NULL,
 [HolidayGroup_id] [int] NULL,
PRIMARY KEY CLUSTERED 
(
 [HolidayID] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, 
ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]
) ON [PRIMARY]

and finally, i am trying to retrieve the Holiday instance using this:

    public static Holiday Get(Int64 _holidayID)
    {
        Holiday _holiday = new Holiday();

        try
        {
            using (var session = Global.NHibernate_SessionFactory.OpenSession())
            {
                _holiday = session.Get<Holiday>(_holidayID); // EXCEPTION OCCURS HERE
            }
        }
        catch (Exception _ex)
        {
            // TODO : Implement proper logging
        }

        return _holiday;
    }

What am i doing wrong?? What's missing? When deleting the table and redefining my object using an Int32 for the ID, everything works! I need use a bigger type!

Many thanks!

EDIT 1 : As Aaronaught mentions, i agree, my need for an Int64 is a bit of overhead for storing holidays… But for a second, let's forget the "Holidays" concept. What am i going to do with my logging table (5 to 10 events (rows) per seconds!). Thanks!

EDIT 2 : @Paco : I'm using NHibernate 2.1.2.4000 and FluentNH 1.0.0.614

EDIT 3 : After rerading Daniel Schilling's solution, rebuilt a (brand new) simple Holiday object that used an Int64 as an ID. I was able to retrieve a record from the DB successfully. As soon as i added a relationship to a Group object, when instanciating a Holiday object i received the same error message as earlier… Here is the HolidayGroup class and mapping, in case you can tell me what i did wrong…

public class HolidayGroup
{
    public virtual int HolidayGroupID { get; set;}
    public virtual string Name { get; set; }
    public virtual string Notes { get; set; }
    public virtual bool IsActive { get; set; }
    public virtual IList<Holiday> Holidays { get; set; }

    public HolidayGroup()
    {
        Holidays = new List<Holiday>();
    }
}

public HolidayGroupMap()
{
Id(x => x.HolidayGroupID);
Map(x => x.Name);
Map(x => x.Notes);
Map(x => x.IsActive);
HasMany(x => x.Holidays)
.Cascade.All()
}