TypeError if given None as a text to index

using repoze.catalog, a text attribute ends up being None, and I get an error while indexing:

Exception happened during processing of request from ('127.0.0.1', 48338)
Traceback (most recent call last):
  File "/home/ccomb/buildout-eggs/Paste-1.7.4-py2.6.egg/paste/httpserver.py", line 1068, in process_request_in_thread
    self.finish_request(request, client_address)
  File "/usr/lib/python2.6/SocketServer.py", line 320, in finish_request
    self.RequestHandlerClass(request, client_address, self)
  File "/usr/lib/python2.6/SocketServer.py", line 615, in __init__
    self.handle()
  File "/home/ccomb/buildout-eggs/Paste-1.7.4-py2.6.egg/paste/httpserver.py", line 442, in handle
    BaseHTTPRequestHandler.handle(self)
  File "/usr/lib/python2.6/BaseHTTPServer.py", line 329, in handle
    self.handle_one_request()
  File "/home/ccomb/buildout-eggs/Paste-1.7.4-py2.6.egg/paste/httpserver.py", line 437, in handle_one_request
    self.wsgi_execute()
  File "/home/ccomb/buildout-eggs/Paste-1.7.4-py2.6.egg/paste/httpserver.py", line 287, in wsgi_execute
    self.wsgi_start_response)
  File "/home/ccomb/buildout-eggs/repoze.browserid-0.3-py2.6.egg/repoze/browserid/middleware.py", line 127, in __call__
    return self.app(environ, start_response)
  File "/home/ccomb/buildout-eggs/Beaker-1.5.4-py2.6.egg/beaker/middleware.py", line 152, in __call__
    return self.wrap_app(environ, session_start_response)
  File "/home/ccomb/buildout-eggs/repoze.zodbconn-0.10-py2.6.egg/repoze/zodbconn/middleware.py", line 15, in __call__
    result = self.application(environ, start_response)
  File "/home/ccomb/buildout-eggs/repoze.tm-1.0a5-py2.6.egg/repoze/tm/__init__.py", line 23, in __call__
    result = self.application(environ, save_status_and_headers)
  File "/home/ccomb/buildout-eggs/repoze.bfg-1.2.1-py2.6.egg/repoze/bfg/router.py", line 130, in __call__
    response = view_callable(context, request)
  File "/home/ccomb/buildout-eggs/repoze.bfg-1.2.1-py2.6.egg/repoze/bfg/configuration.py", line 1753, in _secured_view
    return view(context, request)
  File "/home/ccomb/buildout-eggs/repoze.bfg-1.2.1-py2.6.egg/repoze/bfg/configuration.py", line 1704, in _rendered_view
    response = view(context, request)
  File "/home/dadafkas/projets/virtualcareteam/vctdemo/views/patient_item.py", line 62, in add
    return HTTPFound(location=model_url(pitem, request))
  File "/home/ccomb/buildout-eggs/repoze.catalog-0.7.2-py2.6.egg/repoze/catalog/catalog.py", line 31, in index_doc
    index.index_doc(docid, obj)
  File "/home/ccomb/buildout-eggs/repoze.catalog-0.7.2-py2.6.egg/repoze/catalog/indexes/common.py", line 33, in index_doc
    return super(CatalogIndex, self).index_doc(docid, value)
  File "/home/ccomb/buildout-eggs/zope.index-3.6.0-py2.6-linux-x86_64.egg/zope/index/text/textindex.py", line 50, in index_doc
    self.index.index_doc(docid, text)
  File "/home/ccomb/buildout-eggs/zope.index-3.6.0-py2.6-linux-x86_64.egg/zope/index/text/okapiindex.py", line 228, in index_doc
    count = BaseIndex.index_doc(self, docid, text)
  File "/home/ccomb/buildout-eggs/zope.index-3.6.0-py2.6-linux-x86_64.egg/zope/index/text/baseindex.py", line 97, in index_doc
    wids = self._lexicon.sourceToWordIds(text)
  File "/home/ccomb/buildout-eggs/zope.index-3.6.0-py2.6-linux-x86_64.egg/zope/index/text/lexicon.py", line 66, in sourceToWordIds
    last = element.process(last)
  File "/home/ccomb/buildout-eggs/zope.index-3.6.0-py2.6-linux-x86_64.egg/zope/index/text/lexicon.py", line 175, in process
    for s in lst:
TypeError: 'NoneType' object is not iterable

Indexing None shouldn't be a problem, and should be equivalent to the empty string.