date - null casted to integer?

Hi!

Excuse me, if this been discussed before, but following thing seems to
me a little bit strange:

select '2009-01-12'::date - null::date < '1 day'::interval;
ERROR:  operator does not exist: integer < interval
LINE 1: select '2009-11-12'::date - null::date < '1 day'::interval;
                                               ^
HINT:  No operator matches the given name and argument type(s). You
might need to add explicit type casts.

select ('2009-11-12'::date - null::date)::interval < '1 day'::interval;
ERROR:  cannot cast type integer to interval
LINE 1: select ('2009-11-12'::date - null::date)::interval < '1 day'...

server 8.3.5 It's really not a problem in queries, but could became a
real pain with plpgsql. I'm not sure, but it seems to me that any
operation with null should product null from the standart's point of view?

Thank you beforehand
--
MRJ