Nhibernate error in group by

Question

I’m studying NHibernate and is giving me an error in group by, but at first it is generating me wrong sql, do not know why, follow the code below SQL and its structure.

string hql = "select p.Categoria, count(p) " +
                   "from Produto p " +
                   "group by p.Categoria";

IQuery query = session.CreateQuery(hql);
var resultados = query.List<Object[]>();

And the class structure is like this:

public class Produto 
{
    public virtual int Id { get; set; }
    public virtual string Nome { get; set; }
    public virtual decimal Preco { get; set; }
    public virtual Categoria Categoria { get; set; } 
}

public class Categoria
{
    public virtual int Id { get; set; }
    public virtual string Nome { get; set; }
    public virtual IList<Produto> Produtos { get; set; }
}

The sql generated by Nhibernate:

select
        produto0_.CategoriaId as col_0_0_,
        count(produto0_.Id) as col_1_0_,
        categoria1_.Id as id1_2_,
        categoria1_.Nome as nome2_2_
from
        Produto produto0_
inner join
        Categoria categoria1_
            on produto0_.CategoriaId=categoria1_.Id
group by
        produto0_.CategoriaId

But it is returning me the error below:

'could not execute query [ select product0_.Categoriaid as col_0_0_, Count(product0_.Id) as col_1_0_, categoria1_.Id as id1_2_, categoria1_.Name as nome2_2_ from Product product0_ Inner Join Category categoria1_ on product0_.Categoriaid=categoria1_.Id group by product0_.Categoriaid ] [SQL: select product0_.Categoriaid as col_0_0_, Count(product0_.Id) as col_1_0_, categoria1_.Id as id1_2_, categoria1_.Nome as nome2_2_ from Product product0_ Inner Join Categoria categoria1_ on product0_.Categoriaid=categoria1_.Id group by product0_.Categoriaid]'

In my view, the SQL of group by should also inform the columns of the category, so it is giving error, but I could not find out why it is not generating.