PostgreSQL - ERROR: Cannot cast type character varying to integer

Hello I´m new in the list, and I want to learn a lot about postgresql, now
I´m working in a database and I have this problem:

I have a table called historicoventas as follows (just the structure)

> historicoventas
> (
> codigopedido varchar(20)
> codigohospital varchar(20)
> fecha varchar(15)
> producto varchar(20)
> unidades varchar(10)
> costo varchar(20)
> )
>
And I want to create a view based on that table whit this instruction:

> CREATE OR REPLACE VIEW vw_temphistoricoventas
> (Pedido, Fecha, Hospital, Producto, Unidades, Costo)
> AS
> SELECT codigoPedido , (substring( fecha from 1 for 4 ) || '-'
> || substring( fecha from 5 for 2 ) || '-'
> || substring( fecha from 7 for 2 ) ) AS fecha,
> codigoHospital , producto , sum( unidades::int4 ) AS unidades, sum(
> costo::float4 ) AS costo
> FROM historicoventas
> WHERE ((producto = '759928') OR (producto = '759936') OR
> (producto = '4002382') OR (producto = '4002580'))
> GROUP BY codigoPedido, fecha, codigoHospital, producto
> ORDER BY fecha;
But I always get this message:
> ERROR: Cannot cast type character varying to integer
My developer workstation is under win xp and postgresql 8.0 but in
production server (external web hosting partner) I have a Linux Red Hat with
kernel 2.4.21-27.0.2.ELsmp y PostgreSQL 7.3.
In local machine all castings between data types works fine but in server it
doesn´t.
convertions like sum(unidades::int4) works fine and reading the postgresql
7.3 docs I can see this type of convertions are enabled by the system
Can somebody help me with this topic?

Thanks a lot

>
> Mauricio Fernández A.
> Ingeniero de Sistemas
> Universidad Autónoma de Manizales
> mfacontacto(at)ono(dot)com
> movil: 607365968
>
> P.D. Some historicoventas table rows can be:
> codPedido codHospital fecha producto unidades
> costo
> 0001001299 100058 20050103 759936 1
> 2375
> 0001001299 100058 20050103 759928 1
> 2375
> 0001001300 100078 20050103 759936 1
> 2375
> 0001003044 100086 20050614 759936 1
> 2275.25
> 0001003128 100062 20050623 759936 2
> 4550.5
> 0001003129 100069 20050623 759936 2
> 4550.5
> 0001003131 100019 20050623 759928 5
> 11376.25
> 0001003131 100019 20050623 759936 3
> 6825.75
> 0001003132 100075 20050623 759936 4
> 9101
> 0001003133 100106 20050623 759936 2
> 4550.5
> 0001003134 100031 20050623 759936 2
> 4550.5
> 0001003135 100037 20050623 759936 3
> 6825.75
> 0001003136 100009 20050623 759936 5
> 11376.25
> 0001003137 100009 20050623 759936 1
> 2275.25
> 0001003138 100024 20050623 759936 4
> 9101