Unable to identify an ordering operator '<' for type 'smallint[]'

Hello I have been working in a report based on the query bellow. It works
fine in my development server which is postgres 8.0 but in production I have
postgres 7.3 and I am getting the following error:

SQL Error :
ERROR: Unable to identify an ordering operator '<' for type 'smallint[]'
Use an explicit ordering operator or modify the query

In declaration:
select ac.idanalisis, ac.idpt, pt.codigohospital, ct.nombre as
nombrehospital,
pt.codigomedico, m.nombre || ' ' || m.apellidos as nombremedico,
ac.pacact,
ac.potencialpacientes, ac.planvisitas
from plannegocio.analisiscliente ac
inner join plannegocio.planterritorio pt on ac.idpt = pt.idpt
inner join centrotrabajo ct on pt.codigohospital = ct.codigo
inner join medico m on pt.codigomedico = m.codigo
where ac.idpt in
(select pt.idpt from plannegocio.planterritorio pt where pt.idplan =
1)
group by ac.idanalisis, ac.idpt, pt.codigohospital, ct.nombre,
pt.codigomedico,
nombremedico, ac.pacact, ac.potencialpacientes, ac.planvisitas
order by ct.nombre, nombremedico

The only strange thing in the query is that ac.potencialpacientes is
smallint[] (in fact is the only thing with sense I can see in the error
message) but I am not using the '<' operator as it suggest.

Can somebody help me?.
thanks in advance

Mauricio Fernández A.
Ingeniero de Sistemas
U. Autónoma de Manizales