Trigger cant find function

I seem to be having difficulty creating a trigger. I have creted the
function and tested it which seems to work fine: -

CREATE FUNCTION get_prod_cost_price (varchar, int8) RETURNS float AS '
DECLAREcost FLOAT;
BEGINSELECT cost_price INTO cost FROM product WHERE code = $1;
IF FOUND THEN    UPDATE order_detail SET cost_price = cost WHERE order_detail_id=$2;    RETURN cost;END IF;RETURN 0;
END;
' LANGUAGE 'plpgsql';

dvd=> select get_prod_cost_price ('DVD368', 10027);
get_prod_cost_price
-------------------              9.81
(1 row)



Now I try and create the approprate trigger and I get the following:-

CREATE TRIGGER tg_update_order_detail AFTER insert   ON order_detail FOR EACH ROW   EXECUTE PROCEDURE
get_prod_cost_price('product_id','order_detail_id');
 

ERROR:  CreateTrigger: function get_prod_cost_price() does not exist

It is clear that it does exist so why does the trigger creation code not
find it?

Thanks in advance for any pointers.

Graham