ValueError using lxml when evaluating tags in worker

Observed with maas 1.4+bzr1676+dfsg-0+1689+209~ppa0~ubuntu13.10.1 from the daily PPA on the Garage MAAS:

Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/celery/execute/trace.py", line 181, in trace_task
    R = retval = fun(*args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/provisioningserver/tasks.py", line 365, in update_node_tags
    tags.process_node_tags(tag_name, tag_definition, tag_nsmap)
  File "/usr/lib/python2.7/dist-packages/provisioningserver/tags.py", line 371, in process_node_tags
    batch_size=batch_size)
  File "/usr/lib/python2.7/dist-packages/provisioningserver/tags.py", line 339, in process_all
    partial(try_match_xpath, xpath, logger=logger), node_details)
  File "/usr/lib/python2.7/dist-packages/provisioningserver/utils.py", line 668, in classify
    for ident, subject in subjects:
  File "/usr/lib/python2.7/dist-packages/provisioningserver/tags.py", line 324, in gen_node_details
    yield system_id, merge_details(details)
  File "/usr/lib/python2.7/dist-packages/provisioningserver/tags.py", line 256, in merge_details
    return _details_do_merge(details, root)
  File "/usr/lib/python2.7/dist-packages/provisioningserver/tags.py", line 218, in _details_do_merge
    detail = etree.fromstring(xmldata)
  File "lxml.etree.pyx", line 2993, in lxml.etree.fromstring (src/lxml/lxml.etree.c:62980)
  File "parser.pxi", line 1607, in lxml.etree._parseMemoryDocument (src/lxml/lxml.etree.c:92730)
ValueError: Unicode strings with encoding declaration are not supported. Please use bytes input or XML fragments without declaration.