Mapping Dictionary<int,Driver>

Hey all,

I've really been trying my best to map this particular type of collection but I just can't seem to crack it. I've tried StackOverflow, the newsgroups and this site but none of the code examples I have tried have worked yet :(

Here is the property I am trying to map:

    public virtual IDictionary<int, Driver> StratumSpecificDrivers
    {
        get;
        set;
    }

Here is the mapping override I am using:

    public void Override(FluentNHibernate.Automapping.AutoMapping<Module> mapping)
    {
        mapping.HasManyToMany(x => x.StratumSpecificDrivers)
                    .AsMap<int>(index => index.Column("Stratum").Type<int>(), element => element.Column("DriverId").Type<Driver>())
                    .Table("ModuleDriverOverrides")
                    .Cascade.All();
    }

It might be worthwhile to note that there is also a property in this class that is of the same type but not in a dictionary:

    public virtual Driver DefaultDriver
    {
        get;
        set;
    }

Any help is appreciated!