INOUT/OUT problems with IMMUTABLE

Is the following behavior intended?

CREATE FUNCTION foo(INOUT x integer, INOUT y integer) AS $$
BEGIN   x := x * 10;   y := y * 10;
END;
$$ LANGUAGE plpgsql;

SELECT * FROM foo(1, 2);x  | y  
----+----10 | 20
(1 row)

ALTER FUNCTION foo(integer, integer) IMMUTABLE;

SELECT * FROM foo(1, 2);
ERROR:  function return row and query-specified return row do not match
DETAIL:  Returned row contains 2 attributes, but query expects 0.

A case with a single parameter works:

CREATE FUNCTION bar(INOUT x integer) AS $$
BEGIN   x := x * 10;
END;
$$ LANGUAGE plpgsql;

SELECT * FROM bar(1);bar 
----- 10
(1 row)

ALTER FUNCTION bar(integer) IMMUTABLE;

SELECT * FROM bar(1);bar 
----- 10
(1 row)

-- 
Michael Fuhr
http://www.fuhr.org/~mfuhr/