help with xpath namespace

I am trying to use xpath to extract some information from a XML document.
(if it matters, It's the router config for a Juniper router)

I believe I am having problems with the namespace.
I am trying to extract the serial-number, but I am not getting anything.

Here is the script I have been testing with:

BEGIN;

CREATE TABLE "xml_test" (
data_xml xml
);

INSERT INTO xml_test (data_xml) VALUES ('
<chassis-inventory xmlns="http://xml.juniper.net/junos/9.6R4/junos-chassis">
<chassis junos:style="inventory">
<name>Chassis</name>
<serial-number>JN1111111111</serial-number>
<description>MX960</description>
</chassis>
</chassis-inventory>');

select data_xml from xml_test where data_xml is document;

select (xpath('/chassis-inventory/chassis/serial-number/text()',
data_xml,
ARRAY[ARRAY['junos',
'http://xml.juniper.net/junos/9.6R4/junos-chassis']]
)) from xml_test;

ROLLBACK;

This gives me the following:

BEGIN
CREATE TABLE
INSERT 0 1
data_xml
-------------------------------------------------------------------------------
<chassis-inventory xmlns="http://xml.juniper.net/junos/9.6R4/junos-chassis">+
<chassis junos:style="inventory"> +
<name>Chassis</name> +
<serial-number>JN1111111111</serial-number> +
<description>MX960</description> +
</chassis> +
</chassis-inventory>
(1 row)

xpath
-------
{}
(1 row)

ROLLBACK

Can anyone suggest how I would go about getting the serial-number with xpath?

Thanks