SpQuery orderby lookup field is not working

            using (SPSite site = new SPSite(SPContext.Current.Site.Url))
            {
                using (SPWeb web = site.OpenWeb())
                {
                    SPList lista = web.Lists[list];
                    SPQuery query = new SPQuery();
                    query.Query = String.Format(@"<Where><And><IsNotNull><FieldRef Name='L_fr_praktika' /></IsNotNull><IsNotNull><FieldRef Name='L_fr_subjekt' /></IsNotNull></And></Where><OrderBy><FieldRef Name='L_fr_praktika'/></OrderBy>");
                    query.ViewFields = "<FieldRef Name='L_fr_praktika' /><FieldRef Name='L_fr_subjekt' />";
                    query.ViewFieldsOnly = true;
                    rezults = lista.GetItems(query);
                }
            }

L_fr_praktika is a lookup field.