Using LINQ to find item in a List but get "Value cannot be null. Parameter name: source"

When using LINQ to get data from a list I encounter this error. How can this be fixed?

Value cannot be null. Parameter name: source

var nCounts = from sale in sal
              select new
              {
                  SaleID = sale.OrderID,
                  LineItem = from sli in sale.LineItems
                             group sli by sli.Item into ItemGroup
                             select new
                             {
                                 Item = ItemGroup.Key,
                                 Weeks = ItemGroup.Select(s => s.Week)
                             }
              };

foreach (var item in nCounts)
{
    foreach (var itmss in item.LineItem)
    {
        // MessageBox.Show(itmss.Item.Name);
        itemsal.Add(new Roundsman.BAL.WeeklyStockList(itmss.Item.Name.ToString(),
                    itmss.Item.Code.ToString(),
                    itmss.Item.Description.ToString(),
                    Convert.ToInt32(itmss.Item.Quantity), 2, 2, 2, 2, 2, 2, 2, 2, 2));
    }                      
}

Error which i got after execution my LINQ I got that type of result (one line, orginally):

System.Linq.Enumerable.WhereSelectListIterator<Roundsman.BAL.Sale, <>f__AnonymousType1<int,System.Collections.Generic.IEnumerable <<>f__AnonymousType0<Roundsman.BAL.Items.Item,System.Collections.Generic.IEnumerable <Roundsman.BAL.WeeklyRecord>>>>>