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