ordering by collection count in nhibernate

List<Question> list = nhelper.NHibernateSession
        .CreateQuery("Select q from Question q left join q.Answers a group by q,a order by count(a)")
        .List<Question>();

"column Question.Name is invalid in the select list because it is not contained in either an aggregate function or the GROUP BY clause"

Question.Users.UserId

var sort = from n in c
                       orderby q.Answers.Count
                       select n;