httr - Why does httr fail on the MusicBrainz webservice?

Question

When I visit http://musicbrainz.org/ws/2/artist/?query=artist:Sun+Ra in my web browser I get the kind of response I'm expecting.

But when I

require(httr)
GET('http://musicbrainz.org/ws/2/artist/?query=artist:Sun+Ra') -> sun.ra
content(sun.ra)
>>> NULL

the response body is empty, even though sun.ra got status 200:

Response [http://musicbrainz.org/ws/2/artist/?query=artist:Sun+Ra]
Date: 2014-10-11 17:26
Status: 200
Content-type: application/json; charset=utf-8
  <EMPTY BODY>

Does httr have a problem with "naked" XML responses?

This XML file does not appear to have any style information associated with it.

---

(I actually think I've figured out the answer, just still going to post the question and answer myself in case someone else has the same problem later…)

Answer 1

It's not a problem with XML. Even though MusicBrainz doesn't require an API key, you do need to identify yourself to the service. This is easier seen with RCurl:

require(RCurl)
getURL('http://musicbrainz.org/ws/2/artist/?query=artist:Sun+Ra'
[1] "<?xml version=\"1.0\" encoding=\"UTF-8\"?><error><text>Your requests are
    being throttled by MusicBrainz because the application you are using
    has not identified itself.  
Please update your application, and see http://musicbrainz.org/doc/XML_Web_Service/Rate_Limiting for more information.</text><text>For usage, please see: http://musicbrainz.org/development/mmd</text></error>"