Ошибка при добавлении записей в бд

cmd.CommandText = "insert into [Education] (id_type, id_form, id_extent, id_employee, name_institut, qualification) values (@id_type, @id_form, @id_extent, @id_employee, @name_institut, @qualification)";
                foreach (Education education in List)
                {
                    cmd.Parameters.AddWithValue("@id_type", education.TypEducation);
                    cmd.Parameters.AddWithValue("@id_form", education.FormEducation);
                    cmd.Parameters.AddWithValue("@id_extent", education.ExtentEducation);            
                    cmd.Parameters.AddWithValue("@name_institut", education.Name_institutEducation);
                    cmd.Parameters.AddWithValue("@qualification", education.QualificationEducation);
                    cmd.Parameters.AddWithValue("@id_employee", 1);
                    connection.Open();
                    cmd.ExecuteNonQuery();
                    cmd.Parameters.Clear();
                    connection.Close();
                  }

cmd.Parameters.AddWithValue("@qualification", education.QualificationEducation).Value = "NULL";

cmd.Parameters.AddWithValue("@qualification", education.QualificationEducation ?? DBNull.Value)

if (education.QualificationEducation == null)
                        cmd.Parameters.AddWithValue("@qualification", DBNull.Value);
                    else
                        cmd.Parameters.AddWithValue("@qualification", education.QualificationEducation);