EXCEPT call not working

I have a perl script that interacts with my postgresql server through the
DBI module. When, I send my query to the server it will work fine with
the following:

$sth = $dbh->prepare("
SELECT p.ids_name, m.start_time, m.end_time
FROM mail_schedule m, personnel p
WHERE p.ids_int = m.ids_int AND
m.dow = $weekday
UNION
SELECT p.ids_name, c.on_start, c.on_end
FROM mail_changes c, personnel p
WHERE p.ids_int = c.ids_int
");

But as soon as I place the EXCEPT call in there it stops working and
doesn't return any values:

$sth = $dbh->prepare("
SELECT p.ids_name, m.start_time, m.end_time
FROM mail_schedule m, personnel p
WHERE p.ids_int = m.ids_int AND
m.dow = $weekday
UNION
SELECT p.ids_name, c.on_start, c.on_end
FROM mail_changes c, personnel p
WHERE p.ids_int = c.ids_int
EXCEPT
SELECT p.ids_name, c.on_start, c.on_end
FROM mail_changes c, personnel p
WHERE p.ids_int = c.ids_int
AND c.on_start = 0
AND c.on_end = 0
");

Does anyone have any suggestions as to why my EXCEPT call isn't working?
If you see some drastic or subtle syntax error let me know. I have never
used EXCEPT before, and am not having much luck. thanks.

Chris Mutchler
davron(at)leibnizcreations(dot)com