no value fetch

hi,

Imagine that I have the following table where ts_sent is a timestamp(0)

select * from notification;
to_used | ts_sent | from
---------------------+-------------------------+---------
amanda(at)obs(dot)unige(dot)ch | 2004-07-21 14:19:43+02 | amanda
amanda(at)obs(dot)unige(dot)ch | | postgres

and so, how do I do to fetch the second line by asking :
catch me the line where ts_sent doesn't have a value !

I've try:
select * from notification where ts_sent = null ;
without success...

Anyone has an idea how to do this ?

Thanks in advance !

Cedric BRINER