Invalid attempt to call Read when reader is closed

Question

I got "Invalid attempt to call Read when reader is closed" error. How i resolve it ?

string fetch_isbn = "Select rc.cartdetail_isbn,rc.cartdetail_qty,sb.quantity as old_qty From rt_cartdetail as rc join sell_book as sb on sb.product_id = rc.cartdetail_isbn where rc.cartdetail_cartid='" + Session["cart_pid"] + "'";
SqlCommand objcmd_ISBN = new SqlCommand(fetch_isbn, objconn);
objconn.Open();
{
SqlDataReader reader_isbn = (SqlDataReader)objcmd_ISBN.ExecuteReader();
if (reader_isbn != null && reader_isbn.HasRows)
{
while (reader_isbn.Read())
{
string isbn = reader_isbn["cartdetail_isbn"].ToString();
int qty = Convert.ToInt32(reader_isbn["cartdetail_qty"]);
int old_qty = Convert.ToInt32(reader_isbn["old_qty"]);
int new_qty = 0;
new_qty = old_qty - qty;
reader_isbn.NextResult();
// objconn.Close();
string update_sellqty = "Update sell_book set quantity = '" + new_qty + "' where product_id='" + isbn + "'";
SqlCommand objcmd_update_sellqty = new SqlCommand(update_sellqty, objconn);
//objconn.Open();
objcmd_update_sellqty.ExecuteNonQuery();
// objconn.Close();
}
}
}