Odd behaviour from prefetch_related since upgrading to 1.6

After upgrading to 1.6 yesterday, our production instance is exhibiting some odd behaviour with prefetch_related.

As the example below shows, when using prefetch_related on the foreign field (viewed_cm), the returned instance apparently does not have this foreign field set, even though the earlier filter on the queryset asserted it had to exist. 

>>> cb = user.coursebookmark_set.filter(viewed_cm__course__type=course_type).prefetch_related('viewed_cm').latest('viewed_at')

>>> cb.viewed_cm

Traceback (most recent call last):

  File "<console>", line 1, in <module>

  File "/home/ubuntu/app/ve-web/local/lib/python2.7/site-packages/django/db/models/fields/related.py", line 314, in __get__

    "%s has no %s." % (self.field.model.__name__, self.field.name))

DoesNotExist: CourseBookmark has no viewed_cm.

>>>

When prefetch_related is excluded, everything operates as expected.

>>> cb = user.coursebookmark_set.filter(viewed_cm__course__type=course_type).latest('viewed_at')

>>> cb.viewed_cm

<CourseModule: py3-intro-calculating-things in intro-python-1>

>>>

Has anyone else experienced anything like this before? I can't seem to replicate it not on production.

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