Database query with where clause for column string is in error

[eluser]Unknown[/eluser]
I'm trying to code a query statement to DB2 database on Windows.

My code is

$sql = "select userid, name, surname from adminuser where userid = ? and password = ?";
$query = $this->db->query($sql, array($userid, $password));

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I've got an error like this:

A Database Error Occurred
Error Number:

select userid, name, surname from adminuser where userid = 'user1' and password = 'passw0rd'

Filename: C:\Program Files\Apache Software Foundation\Apache2.2\htdocs\Code\system\database\DB_driver.php

Line Number: 330

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If I copy the SQL statement to query directly with DB2, it works.

I tried querying another table using where clause with integer column, it works fine.

Please help,
Thank you.