Problem with REST call to generate XML

I am trying to fetch some XML data with this REST call, as described in the API docs:
http://www.mockaroo.com/api/generate.xml?key=[my_key] + '&fields=' + [my_JSON];

I am getting this response from the server:
{"error":"Unsupported format xml"}

Data is returned if I use any other format... Json, Csv, and Txt all work. Xml always returns the error. Is there something that I need to do differently to generate XML?

Thanks!!