NULL TIMESTAM problem

Hi! I'm new in Postgres.

I nedd to fill a database table x from a file With the COPY command an the 
delimiter '*'.
This  table  has a timestamp null column (I declared like: ' birthday 
timestamp NULL' ).

But when I try to insert NULL values(specified in the file), postgres throw 
an error.


I don't know how can I specify this NULL value wkthout an '\N' character.


Thanks
Enrique.