xpath

Hi,
I am running postgreSQL-8.4.2. I have a table that stores a single xml
document per row in one of it's fields. I would like to use xpath to
retrieve portions of these xml documents.
Is there a way to do so. (I am running postgreSQL 8.4.2 configured
(built) with --with-libxml and --with-libxslt options)

I have looked at 'xpath' but I am unable to get it work for table fields.

The command below works.
SELECT xpath('/doc/name/@first','<doc><name first="David"
last="Marston"/>...</doc>');

The command below seems not to execute successfully
SELECT a.id,xpath('/doc/name/@first',a.xml_payload) FROM
staging.simple_table a WHERE a.id=1;

HINT: No function matches the given name and argument types. You
might need to add explicit type casts.

Allan.