_text problem in union

Hi,

I have small problem with text array in union query.. see:

abil=> select 5 union select 5;
?column?
--------
5
(1 row)

abil=> select 5 union select 6;
?column?
--------
5
6
(2 rows)

abil=> select '{"aaa"}'::_text union select '{"aaa"}'::_text;
?column?
--------
{"aaa"}
(1 row)

abil=> select '{"aaa"}'::_text union select '{"bbb"}'::_text;
ERROR: Unable to identify an ordering operator '<' for type '_text'
Use an explicit ordering operator or modify the query
abil=>

... hmm, any suggestion?

Zakkr