Exception using Reveal.Member

I get the exception "Could not determine type for: System.Linq.Expressions.Expression1[[System.Func2[[Scala.Map, Scala, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null],[System.Object, mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]], System.Core, Version=3.5.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]], System.Core, Version=3.5.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089, for columns: NHibernate.Mapping.Column(Member)"

When using Id(x => Reveal.Member("id"));

The reason I do this is because I want to keep the database id hidden in my object.

The code:

I have a Map class:

public class Map

{
    /// Database id.
    private int id { get; set; }

public virtual double Height

    {
        get;
        set;
    }

public virtual double Width

    {
        get;
        set;
    }

}

I use the mapping:

public class MapMapping : ClassMap

{
    #region Constructors

    public MapMapping()
    {
        Id(x => Reveal.Member<Map>("id"));

        NaturalId().Property(x => x.Name);

        Map(m => m.Width);
        Map(m => m.Height);
        HasOne(m => m.GeoMapping).Cascade.All();
    }

    #endregion Constructors
}