Group By with nHibernate 2.2

Hi All, and please help me with the following question:

I am working with nHibernate 2.2, VS 2008 and MySQL and

I have a working sql script:

select s.name, count(o.id) from orders o
inner join writers w
    on o.assigned_to = w.id
inner join sites s
    on s.id = w.site_id
where o.date between '2014-09-22 00:00:00' and '2014-09-29 00:00:00'
group by s.name

I am trying to implement it with nHibernate:

var collection = NHibernateManager.Session.CreateCriteria(typeof(Writers.Domain.Order), () => orderAlias)
                .CreateAlias(() => orderAlias.AssignedWriter, () => writerAlias, JoinType.InnerJoin)
                .CreateAlias(() => writerAlias.Site, () => siteAlias, JoinType.InnerJoin)
                .Add(() => writerAlias.RegDate > period.From)
                .Add(() => writerAlias.RegDate < period.To)
                .SetProjection(Projections.ProjectionList()
                    .Add(Projections.GroupProperty(ReflectionHelper.GetMemberName((Site s) => s.Name)))
                    .Add(Projections.Count(ReflectionHelper.GetMemberName((Writers.Domain.Order o) => o.OrderId)), "OrderId")
                ).List();

But I am always getting the following error:

Could not resolve property: Name of Writers.Domain.Order.

Guys, could you please help me with solwing this problem. Thank you!

Askold