var reports = dbContext.Product.ToList();

        if (searchField == "StoreNum")
        {
            int storeNum;
            int? stnum = int.TryParse(searchString, out storeNum) ? storeNum : (int?)null;

           reports = reports
          .Where(w => w.StoreNum == stnum).AsQueryable(); // error occurs here when I try to build

        }