How do I COPY empty Datetimes?

Hi all. I have a problem here. I have to automatize the periodical COPY
of a large set of ASCII data in a postgres database. The parser digests
everything I put in there, with an exception: if there is a DateTime
field that is empty, the copy of the field fails for a "Invalid datetime
format". The field isn't a NOT NULL one, so I have to accept also empty
values... I've tried with the sequence '', "", or simply /t/t (tab is
the field separator in thosa ASCII data), but the output has always been
the same.

I would be immensely grateful to any 1 that could shed some light...
private responses please, I've had to unsubscribe... ya know, the phone
bill!!

TIA