Error in SELECT clause with UPPER function

1) Using
PostgreSQL 7.2.3 on i586-pc-linux-gnu, compiled by GCC 2.95.3

2) Description

track=# select loginname, upper(loginname) from tperson;
loginname | upper
------------+------------
admin | ADMIN
prempena | PREMPENA
(2 rows)

track=# select loginname, upper(loginname) from tperson where
upper(loginname)='ADMIN';
loginname | upper
-----------+-------
(0 rows)

3) Expected result
loginname | upper
------------+------------
admin | ADMIN
(1 row)

If someone can help to understand why …

Thanks in advance.

Pascal