SELECT and DATE Function question

Hi,

I'm trying to calculate an expiration date by adding the number of days onto
the start date. i.e. select start_date + number_of_days from blah;

create table blah (start_date timestamp, number_of_days integer);
insert into blah values (current_timestamp, 25);
select start_date + number_of_days from blah;

The error I get is:

ERROR: operator does not exist: timestamp without time zone + integer
HINT: No operator matches the given name and argument type(s). You may need
to add explicit type casts.

But according to
http://www.postgresql.org/docs/8.1/static/functions-datetime.html the +
operator should support integers and treat them as days (date '2001-09-28' +
integer '7'). Obviously typing a constant into the query is a lot different
from using the value of a column, but I would have thought it would work.

What is the correct way to make this calculation?

Cheers,

Mike