Bad timestamp external representation

Hello everybody

I meet the "bad timestamp external representation" error, and I don't know
how to solve it. I tried, of course, to look for documentation myself, but I
feel lost with this... any help will be greatly appreciated.
I first met the problem with a view:

create view allprod as
(
SELECT equip,produit,nserie,exserie,ech FROM package
union
SELECT id,produit,nserie,exserie,ech FROM equip
union
SELECT equip_id,'',produit,'','' FROM cles
)
The view is created succesfully, but if I try a "select * from allprod", I
have the timestamp error.
"ech" is the timestamp field, it can be null in some rows. If I use it in
only one of the unions (instead of 2), the same problem occur. Of course, if
I create the view without the "ech" field, I can select or do what I want
without trouble.
I also found this: on a normal table with a timestamp field (called, say,
"tstamp"), I will get the error too with a query like "select * from
my_table where tstamp=''", and no error with "...where tstamp is null".
Well, it may be easy for advanced user, but not for me !
I use postgresql 7.1.3 on a Debian 2.2r3 system, with a "fr_FR" locale.
Thanks in advance

Francois Thomas