Unable to convert null timestamp to date. Bug?

Strange, this works:
  select * from users where last_visit > now() + 7; 
  -- last_visit is nullable, of type timestamp

But this doesn't
  select * from users where last_visit + 7 > now();
  ERROR:  Unable to convert null timestamp to date
  -- yes, there are users where last_visit IS NULL

BTW, this works too:
  select null+1; -- obviously returns NULL

System is FreeBSD 4.1, PgSQL 7.0.2

Anyone got a clue?

Yours sincerely,

Edmar Wiggers
BRASMAP Information Systems
+55 48 9960 2752