Converting a timestamp to a date when it contains nulls.

Hi.
I'm not sure if the following behaviour is expected or not.

Version info:
psql (PostgreSQL) 7.0.1
4.0-STABLE FreeBSD 4.0-STABLE #0: Tue Apr 25 18:34:52 EDT 2000

I have a table:
create table test (td timestamp);
I add a row:
insert into test values ('now');
I select from it casting td to a date:
select td::date from test;
  ?column?  
------------
 2000-07-23
(1 row)

Everything is peachy.

Now I insert a NULL:
insert into test values (NULL);
Everything is no longer peachy.
select td::date from test;     
ERROR:  Unable to convert null timestamp to date

Shouldn't all casts be able to handle the case where it is NULL?

-Michael