Error: XXX is not JSON serializable

I have an app I'm moving from local dev (mac, python 2.7.2, django 1.5.4) to WebFaction (python 2.7.5.

I'm storing record ids like so:

section_ids = list(sections.values_list('id', flat=True))
request.session['query.section_ids.last'] = section_ids


works fine locally and fails on WF with:

(as an example)

    [10892L, 10891L] is not JSON serializable  

This is the traceback...

File "/home/leedhinde/webapps/myapp/lib/python2.7/django/core/handlers/base.py" in get_response

  187.                 response = middleware_method(request, response)

File "/home/leedhinde/webapps/myapp/lib/python2.7/django/contrib/sessions/middleware.py" in process_response

  38.                     request.session.save()

File "/home/leedhinde/webapps/myapp/lib/python2.7/django/contrib/sessions/backends/db.py" in save

  51.             session_data=self.encode(self._get_session(no_load=must_create)),

File "/home/leedhinde/webapps/myapp/lib/python2.7/django/contrib/sessions/backends/base.py" in encode

  84.         serialized = self.serializer().dumps(session_dict)

File "/home/leedhinde/webapps/myapp/lib/python2.7/django/core/signing.py" in dumps

  100.         return json.dumps(obj, separators=(',', ':')).encode('latin-1')

File "/usr/local/lib/python2.7/json/__init__.py" in dumps

  250.         sort_keys=sort_keys, **kw).encode(obj)

File "/usr/local/lib/python2.7/json/encoder.py" in encode

  207.         chunks = self.iterencode(o, _one_shot=True)

File "/usr/local/lib/python2.7/json/encoder.py" in iterencode

  270.         return _iterencode(o, 0)

File "/usr/local/lib/python2.7/json/encoder.py" in default

  184.         raise TypeError(repr(o) + " is not JSON serializable")

I'm not clear why this wouldn't work. I also haven't figured out testing session updating in the shell for simpler testing.

Any pointers would be appreciated.

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