ERROR: using an explicit cast

Hi all

I am using postgres 6.3.2 and I experience problems using a union on views
In short, I have the following views defined:

create view vstockpart as select 'StockPart' as vpisource, pspid as
vpisourceid;
create view vquoteoutitem as select 'QuoteOutItem' as vpisource, fqoiid as
vpisourceid;

where pspid and fqoiid are both of type int.

When I launch the following query:

select vpisource,vpisourceid from vstockpart union select
vpisource,vpisourceid from vquoteoutitem;

then I get the following error:

NOTICE:    there is no more then one operator < for types
NOTICE:    unknown and unknown. You will have to retype this query
ERROR:    using an explicit cast.

Where can I find documentation regarding casts?
Does anybody know what is wrong? Or do I have to move to 6.4?

TIA
Wim Ceulemans - wim.ceulemans@nice.be
Nice Software Solutions
Eglegemweg 3, 2811 Hombeek - Belgium
Tel +32(0)15 41 29 53 - Fax +32(0)15 41 29 54