NVL behaving differently on two instances of Oracle 9.2

Question

Hello,
one of our customers is running two Oracle 9.2.0.6 64Bit instances. One for development and one for production usage of our software.
The following statement works on the development instance:
SELECT nvl(content, '')
FROM a_table;
The column content is a CLOB. The same statement crashes in production with the message "inconsistent data types. got char, expected clob.".
The fix is fairly easy.
SELECT nvl(content, to_clob(''))
FROM a_table;
This works on both instances.
I asked their DBA if there are different compatibility settings for the instances but they both are "9.2". Although the statement works now, we and they would like to know what the reason for this behavior could be.

Answer 1

Are you sure the code is same in both instances?
Notice one single space for the secodn argument in the second select.
Also, your '' evaluates to a NULL anyway, so I do not see a reason to be using NVL to convert a NULL back into a NULL.
SQL> desc t
Name                                      Null?    Type
CONTENT                                            CLOB
SQL> set null null
SQL> SELECT nvl(content, '') from t ;
NVL(CONTENT,'')
*$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$
null
null
3 rows selected.
SQL> SELECT nvl(content, ' ') from t ;
SELECT nvl(content, ' ') from t
ERROR at line 1:
ORA-00932: inconsistent datatypes: expected CLOB got CHAR
SQL> disconnect
Disconnected from Oracle9i Enterprise Edition Release 9.2.0.7.0 - Production
With the Partitioning, OLAP and Oracle Data Mining options
JServer Release 9.2.0.7.0 - Production
SQL>