Problem with group by command

Dear PGSQL-BUG group,

I would be grateful if any of you could provide me any information that you
may have.

I have written a perl program which runs fine on a machine that was using
PostgreSQL 6.4.2. I setup another machine which was running PostgreSQL
6.5.3 bundled by the Linux 6.2. I used the copy command to migrate most of
the tables on the new machine and re-run the same program.

The same error message keeps on appear said that:
Invalid use of aggregates or non-group column in target list.

The SQL statement used is:
select username, user_index, date, status from subscribers, cellnet_mesg_log
where status != 3 and date >= '25/10/2000' and date < '01/11/2000' and
cellnet_mesg_log.user_index = subscribers.oid group by username;

The 'group by' works in the new machine only if the selected data contains
username which is so weird. I wonder whether some of the attributes might
have been changed since I did not restore the same tables from pg_dumpall
command.
If that is the case, I wonder if I could manually compare and reset the
attributes.
Unfortunately, I couldn't find this information from your web page.

The tables structure are as below. The cellnet_mesg_log's user_index is map
with the oid of the subscribers table.

Table = subscribers
+----------------------------------+----------------------------+
|Field |Type | Length|
+----------------------------------+--------------------+-------+
| username | varchar() | 32 |
| acct_no | int4 | 4 |
| acct_type | int4 | 4 |
| build_factor | int4 | 4 |
+----------------------------------+-----------------------------

Table = cellnet_mesg_log
+----------------------------------+----------------------------------+-----
--+
| Field | Type |
Length|
+----------------------------------+----------------------------------+-----
--+
| user_index | oid |
4 |
| date | datetime |
8 |
| sender | varchar() |
64 |
| part | int4 |
4 |
| part_total | int4 |
4 |
| status | int4 |
4 |
| mesg_id | varchar() |
64 |
| message | varchar() |
160 |
+----------------------------------+----------------------------------+-----
--+

Hope to hear from any of you soon. Thank you for your kind attention.

regards,
Jessica Ord