NHibernate free order on a property in a linked object using QueryOver

I have itemgroup items and tables in my database:

CREATE TABLE [items](
    [item_id] [int] IDENTITY(1,1) NOT NULL,
    [item_name] [varchar](50) NOT NULL,
    [group_id] [int] NOT NULL   
)

CREATE TABLE [itemgroup](
    [group_id] [int] IDENTITY(1,1) NOT NULL,
    [group_name] [varchar](50) NULL 
)

and here are the mapping classes for these objects:

public class ItemMap : ClassMap<Item>
{
    public ItemMap()
    {
        Table("items");
        Id(x => x.Id).Column("item_id");
        Map(x => x.Name).Column("item_name");
        References(x => x.ItemGroup).Column("group_id").Fetch.Join();            
    }
}
public class ItemGroupMap : ClassMap<ItemGroup>
{
    public ItemGroupMap()
    {
        Table("itemgroup");
        Id(x => x.Id).Column("group_id");
        Map(x => x.Name).Column("group_name");
    }
}

How can I get all items from a database sorted by group name using QueryOver?

I know how to do this using criteria (using an alias).

var criteria = Session.CreateCriteria<Item>()
                      .CreateAlias("ItemGroup", "group")
                      .AddOrder(Order.Asc("group.Name"));

I tried to create an alias ItemGroup and use it with QueryOver, but my result was sorted by item name, not by item group name.