how to create this trigger?

I need to create a trigger that increment a value in a column 
of table1 where a row is inserted in table 2

I have tried whit this but it just does nothing.

CREATE FUNCTION increment_value () RETURNS opaque AS 
'DECLARE  code int4;  BEGIN  code := new.code;  UPDATE table1   SET value = value + 1  WHERE id = code;
RETURN NEW;
END;'  LANGUAGE 'plpgsql';

CREATE TRIGGER insert_on_table2 BEFORE INSERT ON table2 FOR EACH ROW EXECUTE PROCEDURE increment_value();


Can you help me?