How do I use the Discoverer module with pygi GstPbutils?

Question

I'm trying to port some pygtk music player code to pygi which uses gst's discoverer module.

from gi.repository import Gst, GstPbutils

def on_discovered(discoverer, ismedia):
    print("%s -- %s" %( discoverer.tags.get('title', 'Unknown'),
                        discoverer.tags.get('artist', 'Unknown')))

Gst.init(None)
location = "file:///srv/Music/molly_hatchet-the_creeper.mp3"
discoverer = GstPbutils.Discoverer()
discoverer.discover_uri(location)
discoverer.connect('discovered', on_discovered)

When I attempt to run this I get the following error:

/usr/lib/python2.7/dist-packages/gi/types.py:43: Warning: g_object_get_qdata: assertion `G_IS_OBJECT (object)' failed
  return info.invoke(*args, **kwargs)
/usr/lib/python2.7/dist-packages/gi/types.py:43: Warning: g_object_ref_sink: assertion `G_IS_OBJECT (object)' failed
  return info.invoke(*args, **kwargs)

** (python:21482): CRITICAL **: pygobject_register_wrapper: assertion `PyObject_TypeCheck(self, &PyGObject_Type)' failed
/usr/lib/python2.7/dist-packages/gi/types.py:43: Warning: g_object_unref: assertion `G_IS_OBJECT (object)' failed
  return info.invoke(*args, **kwargs)

Unfortunately documentation on this pygi module seems a bit sparse.

For now I've reverted back to pygtk, but I'd love to know how to make this work with pygi since that's going to be the way forward with Ubuntu.