Local Web Service

I've created a local web service based on core_enrol_get_enrolled_users. It works almost perfectly, for some reason the Username field shows up as:

<KEY name="username">

      <VALUE null="null"/>

</KEY>

This is a REST request, on Moodle 2.3. I wondered if I had just missed something but if I use the token to call core_enrol_get_enrolled_users it does the same thing. Is this a permissions problem? Maybe my WSuser isnt' allowed to view usernames?

Thanks for any help.