(2) failed to find conversion function from "unknown" to ...

Hi!

Other (besides text literal from my previous message)
condition when we have
ERROR: failed to find conversion function from "unknown" to ..
is usage of null.

When I invoke

select 1 where 5 in (null)

everything is Ok,
though when I try

select 1 where 5 in (select null)

I get

ERROR: failed to find conversion function from "unknown" to integer.

Is it desired behavior or subject to change
in future versions of PG?

Thanks.

--
Best regards
Ilja Golshtein