getting custom variables from chat REST api

i'm trying to get the custom variables i had set from the chat REST api.

i do a GET request to

https://dev.liveperson.net/api/account/<my account number>/chat/<my chat session id>/customVariables?v=1

as described here:

https://community.liveperson.com/community/developers/product?view=chatapirest

with these headers:

Accpet: application/xml

Content-Type: application/xml

Authorization: LivePerson appKey=<my app key>

the reply I get is

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>

<error>

    <time>2016-01-11T10:14:52.560-05:00</time>

    <message></message>

    <internalCode>28</internalCode>

</error>

what does InternalCode 28 mean?  And what am I doing wrong?  How can I get the custom variables?