exception while upgrading driver

Hi
i have the following code running without any errors with postgres 7.4.2.
driver.

String qry="select MAX(num) from (select
to_number((substr(acr_id,4)),'9999') as num from aircraft where acr_id like
'ACR%' group by acr_id) as num "; // order by num desc";

try{
ResultSet rs=Data.getResultSet(qry);
while(rs.next())
{
try{
acrmax=Integer.parseInt(rs.getString(1))+1;
}catch(NumberFormatException ne){acrmax=0;}
}
}
catch(SQLException ex) { System.err.println("for rs1 err
"+ex.getMessage());
}

I changed the driver to postgres 8.0.1. Strangely now, the above code gives
the exception "The resultset is closed". Please help.

regards,

Sathya