using a generated series in function

Hi,

I am attempting (without success) use the generated series of dates that come
from:
select (cast(date_trunc('week', '2011-11-20'::date ) as date) + (i+6)) as
week_date from generate_series(0,84,7) i

in a function.
select function_name(integer, date); -- function returns a numeric

This does NOT work:
select (function_name(303, week_date::date)) as week_date where week_date in
(select (cast(date_trunc('week', '2011-11-20'::date ) as date) + (i+6)) as
week_date from generate_series(0,84,7) i )

The error is:
ERROR: column "week_date" does not exist
LINE 1: select (xchromasun._chromasun_getqtyordered(303, week_date::...

I hope I can do this? What am I doing wrong?
Johnf