LINQ: System.Int32 - это тип значения, не допускающий значения NULL

Ошибка: значение NULL не может быть присвоено члену с типом System.Int32, который не допускает значения NULL. Программа аварийно завершает работу. здесь:

Nullable<Int32> maxTagFrequency = (from t in tagSummary select t.tagCount).Max();

Это странно, потому что я объявил переменную допускающей значение NULL, int? maxTagFrequency тоже не работает ...

Весь запрос LINQ:

private void BindTagCloud()
{

 int pro_id = Convert.ToInt32(proj_id);

    var tagSummary = from af in db.AgileFactors
               join psf in db.ProjectStoryFactors on af.AgileFactorID equals psf.AgileFactorID
               join s in db.Stories on psf.StoryID equals s.StoryID 
               join pim in db.ProjectIterationMembers on s.ProjectIterationMemberID equals pim.ProjectIterationMemberID
               join it in db.Iterations on pim.ProjectIterationID equals it.ProjectIterationID
               join pro in db.Projects on it.ProjectID equals pro.ProjectID
               where pro.ProjectID == pro_id &&
                     pro.ProjectID == it.ProjectID &&
                     it.ProjectIterationID == pim.ProjectIterationID &&
                     pim.ProjectIterationMemberID == s.ProjectIterationMemberID &&
                     s.StoryID == psf.StoryID &&
                     psf.AgileFactorID == af.AgileFactorID
                     group af by af.Name into tagGroup

                     select new
                     {

                        Tag = tagGroup.Key,
                        tagCount = tagGroup.Count()

                     };

    Nullable<Int32> maxTagFrequency = (from t in tagSummary select t.tagCount).Max();

var tagCloud = from af in db.AgileFactors
                   join psf in db.ProjectStoryFactors on af.AgileFactorID equals psf.AgileFactorID
                   join s in db.Stories on psf.StoryID equals s.StoryID
                   join pim in db.ProjectIterationMembers on s.ProjectIterationMemberID equals pim.ProjectIterationMemberID
                   join it in db.Iterations on pim.ProjectIterationID equals it.ProjectIterationID
                   join pro in db.Projects on it.ProjectID equals pro.ProjectID
                   where pro.ProjectID == pro_id &&
                         pro.ProjectID == it.ProjectID &&
                         it.ProjectIterationID == pim.ProjectIterationID &&
                         pim.ProjectIterationMemberID == s.ProjectIterationMemberID &&
                         s.StoryID == psf.StoryID &&
                         psf.AgileFactorID == af.AgileFactorID
                   group af by af.Name into tagGroup
                   select new
                   {

                       Tag = tagGroup.Key,
                       weight = (double)tagGroup.Count() / maxTagFrequency * 100
                   };

  ListView1.DataSource = tagCloud; 
  ListView1.DataBind();

}