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