Unable to identify an ordering operator

Hello,

select persons.name,firm.name,persons.tel
from persons,work,firm
where (persons.table_owner=0) and
(work.id_firm=firm.id and work.id_person=persons.id)
union
select persons.name,firm.name,persons.tel
from persons,work,firm
where (persons.table_owner=1) and
(work.id_firm=firm.id and work.id_person=persons.id);

ERROR: Unable to identify an ordering operator '<' for type '_text'
Use an explicit ordering operator or modify the query

I know that this query can be rewrite to eliminate the UNION using an
OR operator on first condition but my question is:

How can I specify an ordering operator? On which field have I to put it?

Best wishes for the new YEAR, \fer