SQL & PL/SQL

Hi.
I am developing a web application (with ASP.NET, framework version 1.1)
which has to run some queries on a Oracle server (version 9.2), through
System.Data.OracleClient.OracleDataAdapter.

When I run a particular query, I retreive the following error through
the .NET driver
"ORA-01405: fetched column value is NULL".


The SQL query that raises the problem is something like that
"SELECT [...],
RIGHE_OFF.IS_CANONE As Canone,
[...]"


Moreover, if I run the same query with TOAD (sql client for Oracle), no
exception is raisen, even if some NULL data is effectively returned.


After several hours wihtout any idea, I found a "dirty" solution to fix
the problem, and now my query is
"SELECT [...],
CASE WHEN RIGHE_OFF.IS_CANONE IS NULL THEN
NULL
ELSE
RIGHE_OFF.IS_CANONE
END As Canone,
[...]"


The query finally returns what I need, but my solution is just a
workaround on the problem, because, even looking for informations on
the net, I coudn't understand the problem.
And, above all, I don't understand why, with such a fool artifice, I
could solve the problem!


Can someone tell me how to avoid that stupid trick, or at least why I
have to do it?
Thanx in advance!
Alessio