Исключение типа "Заданное приведение является недопустимым"

Хочу взять данные из двух столбцов БД, выполнить с ними расчеты и занести в график.

string sql = "Select * From Детали";

db.OpenConnection();
SqlCommand command = new SqlCommand(sql, db.getConnection());
SqlDataReader reader = command.ExecuteReader();

while (reader.Read())
{
    int type = reader.GetInt32(3);
    int kol = reader.GetInt32(6);

    int sum = type * kol;

    cartesianChart1.Series = new SeriesCollection
    {
        new LineSeries
        {
            Values = new ChartValues<ObservablePoint>
            {
                new ObservablePoint (0, sum),
                new ObservablePoint (2, sum),
                new ObservablePoint (4, sum),
                new ObservablePoint (6, sum),
                new ObservablePoint (8, sum),
                new ObservablePoint (8, sum),
                new ObservablePoint (8, sum),
                new ObservablePoint (8, sum),
                new ObservablePoint (8, sum),
                new ObservablePoint (8, sum)
            },
            PointGeometrySize = 15
        }
    };
}