Error while dispatching download

[miken32]

I start my servers like so, and can render articles no problem:

mw-qserve -p 14311 -i 127.0.0.1 &
nserve --port 8899 --qserve 127.0.0.1:14311 &
nslave --serve-files-port 8898 --cachedir /var/cache/mwlib/ &

But I get the following output when trying to download the rendered PDF:

ERROR while dispatching 'download': {'qserve': ('localhost', 14311), 'collection_id': '4a3642d5dcbcbc22', 'is_new': False}
Traceback (most recent call last):
  File "/usr/local/python27/lib/python2.7/site-packages/mwlib/nserve.py", line 238, in dispatch
    return method(collection_id, request.params, is_new)
  File "/usr/local/python27/lib/python2.7/site-packages/mwlib/nserve.py", line 384, in do_download
    download_url = res["result"]["url"]
KeyError: 'result'
Traceback (most recent call last):
  File "/usr/local/python27/bin/bottle.py", line 862, in _handle
    return route.call(**args)
  File "/usr/local/python27/bin/bottle.py", line 1732, in wrapper
    rv = callback(*a, **ka)
  File "/usr/local/python27/lib/python2.7/site-packages/mwlib/nserve.py", line 171, in dispatch_command
    return Application().dispatch(request)
  File "/usr/local/python27/lib/python2.7/site-packages/mwlib/nserve.py", line 243, in dispatch
    raise exc
KeyError: 'result'

Is this a known problem or a bug? I checked the cache directory for that collection ID and the PDF does exist at /var/cache/mwlib/4a/4a3642d5dcbcbc22/output.rl.