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@obs.unige.ch |  2004-07-21 14:19:43+02 | amanda
 amanda@obs.unige.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