update-apt-xapian-index crashes due to software-center plugin

Binary package hint: software-center

software-center version 2.1.14.1

This crash only occurs if a package is encountered that defines custom metadata fields as defined for new apps on a stable release. To reproduce, just run update-apt-xapian-index from the command line. It crashes with the following output:

$ sudo update-apt-xapian-index -f
Reading .desktop files from /usr/share/app-install/desktop/: done.
Rebuilding Xapian index... 7%
Traceback (most recent call last):
  File "/usr/sbin/update-apt-xapian-index", line 104, in <module>
    indexer.rebuild(opts.pkgfile)
  File "/usr/lib/pymodules/python2.6/axi/indexer.py", line 681, in rebuild
    self.buildIndex(dbdir, generator)
  File "/usr/lib/pymodules/python2.6/axi/indexer.py", line 656, in buildIndex
    for doc in documents:
  File "/usr/lib/pymodules/python2.6/axi/indexer.py", line 538, in gen_documents_apt
    yield self.get_document_from_apt(pkg)
  File "/usr/lib/pymodules/python2.6/axi/indexer.py", line 506, in get_document_from_apt
    addon.obj.index(document, pkg)
  File "/usr/share/apt-xapian-index/plugins/software-center.py", line 103, in index
    if CUSTOM_KEY_THUMBNAIL_URL in ver.record:
NameError: global name 'CUSTOM_KEY_THUMBNAIL_URL' is not defined