Trigger won't execute when using sp with input parameters

Hi!

I'm not sure if this has anything to do with the npgsql data provider.
I'm trying to implement tsearch2 functionality within a table.
Each time a row is inserted or updated the following trigger should
execute:

CREATE TRIGGER tsvectorupdate
BEFORE INSERT OR UPDATE
ON "public".object
FOR EACH ROW
EXECUTE PROCEDURE tsearch2(idxfti, header, description);

Insertions are made from an ASP.NET page using npgsql.

When using an ''INSER INTO' SQL statement the trigger is executing fine.
Also a stored procedure with "hard coded" values will fire the trigger.

myCommand.CommandText = "SELECT
insert_object('book',61,'ABC','DEF',110,'S','abc(at)123(dot)com')";

But when using input parameters (and a output parameter) the trigger
won't execute.

myCommand.CommandType = CommandType.StoredProcedure;
myCommand.CommandText = "SELECT
insert_object(:db_table_destination,:category_id,:header,:description,:price,:type_of_ad,:person_id)";

Below is the stored procedure:

CREATE OR REPLACE FUNCTION insert_object("varchar", int4, "varchar",
"varchar", "numeric", "varchar", "varchar")
RETURNS int4 AS
$BODY$DECLARE
insertString text;
id int4;

BEGIN
insertString := 'INSERT INTO ' || _db_table_destination
|| ' (category_id, header, description, price,
created_on, type_of_ad, person_id) '
|| 'VALUES (' || quote_literal(_category_id) || ','
|| quote_literal(_header) || ','
|| quote_literal(_description) || ','
|| quote_literal(_price) || ','
|| 'DEFAULT' || ','
|| quote_literal(_type_of_ad) || ','
|| quote_literal(_person_id) || ');';

--RAISE NOTICE '%', insertString;
EXECUTE insertString;

id := currval('object_id_seq');

RETURN id;

END;
$BODY$
LANGUAGE 'plpgsql' VOLATILE;
ALTER FUNCTION insert_object("varchar", int4, "varchar", "varchar",
"numeric", "varchar", "varchar") OWNER TO postgres;

Any ideas?
Kind regards, Erik