Return Opaque / Error in Function

The following function is executed by a trigger. I'am receiving the error
message following. What am I doing wrong ? Can someone help me ?

Create Function update_productvariant()
RETURNS OPAQUE
AS 'DECLARE
pid text;
BEGIN
pid := old.productvariantid;
Insert into productupdate (productvariantid) values (pid);
RETURN old;
END;'
Language 'plpgsql';


Error:
mag=# Delete from productvariant where productvariantid='4015223929157';
ERROR:  control reaches end of trigger procedure without RETURN
DEBUG:  Last error occured while executing PL/pgSQL function
update_productvariant
DEBUG:  at END of toplevel PL block
ERROR:  control reaches end of trigger procedure without RETURN