Unable to identify a right operator '>=' for type 'bpchar'

Hi,

While running the following SQL statement in postgresql-7.0-1 via
JDBC,
  select a.cus_code, a.dlv_code, b.cus_abbr, a.dlv_abbr,         a.address, a.tel, a.fax, a.contact    from dlv_point
a,customer b   where ((a.cus_code >= ? and a.cus_code <= ?)         or (b.cus_abbr >= ? and b.cus_abbr <= ?))     and
trim(b.cus_code)= trim(a.cus_code)   order by 1 ;
 

I got the following error message:
  Unable to identify a right operator '>=' for type 'bpchar'  You may meed to add parentheses or an explicit cast.

I have never got this kind of message from Informix or Oracle.

The data type of the fields in the above statement are all char(n).

Can anyone help?

The system I am using is Red Hat Linux 6.2 with Kernel 2.2.14-6.1.1 on
an i586

Thanks in advance.

Michael