hstore problem with UNION?

I've encountered the following problem:

ivoras=# create table htest2(id integer, t hstore);
CREATE TABLE

ivoras=# create table htest3(id integer, t2 hstore);
CREATE TABLE

ivoras=# select id, t from htest2 union select id,t2 as t from htest3;
ERROR:  could not identify an ordering operator for type hstore
HINT:  Use an explicit ordering operator or modify the query.

I think it has something to do with UNION being the type of operation
that could, as a variation, include DISTINCT, which would require
sorting, but still... UNION by itself doesn't.

How to get around this? I really don't care how hstores get sorted and
more, would like to avoid sorting them at all as they could get big.