again, pgplsql and parameters question

Soory for this retry, here it is

With pg 7.1.3,

I don't get parameters working in my pgplsql functions.
I.e.:

CREATE FUNCTION add_one (INTEGER) RETURNS INTEGER AS '
BEGIN
RETURN ($1 + 1);
END;
' LANGUAGE 'plpgsql';

Gives:
ERROR: Unable to identify left operator '+' for type 'int4'
You may need to add parentheses or an explicit cast

While the following one works just fine.

CREATE FUNCTION startweek() RETURNS DATE AS '
DECLARE
start date;
BEGIN
start:= to_date(''01-01-''||date_part(''year'', timestamp ''now''),''DD
MM YYYY'') + date_part(''week'', timestamp ''now'') * 7 - 8;
RETURN start;
END;
' LANGUAGE 'plpgsql';

ERROR: Unable to identify left operator '+' for type 'int4'
You may need to add parentheses or an explicit cast

Any idea?
Thanks,
Alex