Cant find table 0 : The code works great on other tables.
The code return true if he found on the table the thing he get and false he hi did not find it.
Error Translation: cant find table 0

            public bool Found(long num)
        {
            DataSet ds = new DataSet();
            string str = string.Format("select * from Customers where Customer_Id={0} ", num);
            ds = ReturnDS(str);
            if (ds.Tables[0].Rows.Count == 0)
                return false;
            else
                return true;
        }

[The Error&Code][1]
  [1]: https://i.stack.imgur.com/6nxgk.png