NameError on failure to find custom properties

The following traceback is raised when the schema-image.json file cannot be found:

Traceback (most recent call last):
  File "bin/glance-api", line 53, in <module>
    app = config.load_paste_app()
  File "/opt/stack/glance/glance/common/config.py", line 179, in load_paste_app
    app = deploy.loadapp("config:%s" % conf_file, name=app_name)
  File "/usr/lib/python2.7/dist-packages/paste/deploy/loadwsgi.py", line 247, in loadapp
    return loadobj(APP, uri, name=name, **kw)
  File "/usr/lib/python2.7/dist-packages/paste/deploy/loadwsgi.py", line 272, in loadobj
    return context.create()
  File "/usr/lib/python2.7/dist-packages/paste/deploy/loadwsgi.py", line 710, in create
    return self.object_type.invoke(self)
  File "/usr/lib/python2.7/dist-packages/paste/deploy/loadwsgi.py", line 203, in invoke
    app = context.app_context.create()
  File "/usr/lib/python2.7/dist-packages/paste/deploy/loadwsgi.py", line 710, in create
    return self.object_type.invoke(self)
  File "/usr/lib/python2.7/dist-packages/paste/deploy/loadwsgi.py", line 144, in invoke
    **context.local_conf)
  File "/usr/lib/python2.7/dist-packages/paste/deploy/util.py", line 56, in fix_call
    val = callable(*args, **kw)
  File "/usr/lib/python2.7/dist-packages/paste/urlmap.py", line 25, in urlmap_factory
    app = loader.get_app(app_name, global_conf=global_conf)
  File "/usr/lib/python2.7/dist-packages/paste/deploy/loadwsgi.py", line 350, in get_app
    name=name, global_conf=global_conf).create()
  File "/usr/lib/python2.7/dist-packages/paste/deploy/loadwsgi.py", line 710, in create
    return self.object_type.invoke(self)
  File "/usr/lib/python2.7/dist-packages/paste/deploy/loadwsgi.py", line 146, in invoke
    return fix_call(context.object, context.global_conf, **context.local_conf)
  File "/usr/lib/python2.7/dist-packages/paste/deploy/util.py", line 56, in fix_call
    val = callable(*args, **kw)
  File "/opt/stack/glance/glance/common/wsgi.py", line 376, in factory
    return cls(routes.Mapper())
  File "/opt/stack/glance/glance/api/v2/router.py", line 36, in __init__
    custom_image_properties = images.load_custom_properties()
  File "/opt/stack/glance/glance/api/v2/images.py", line 332, in load_custom_properties
    LOG.warn(msg % filename)
NameError: global name 'filename' is not defined