strange behaviour in plpgsql:null arguments

Hi, I think I found something that is strange.
I can't detect whether i passed a NULL to stored procedure
in plpgsql when it happens that I use composites
I warn that I haven't read the sql standards though.
Here is the code:

CREATE TYPE ttype AS (
t1 int,
t2 int
);

CREATE OR REPLACE FUNCTION ttypetest(t ttype) RETURNS ttype AS $$
BEGIN
IF t IS NULL THEN
RAISE EXCEPTION 'NULL input';
END IF;
RETURN t;
END
$$ LANGUAGE plpgsql;

SELECT ttypetest(NULL);

I also find it quite difficult to unset an element from array in postgres.

Regards,
Peter Filipov