Concurrent switch_db failure

I have multiple green threads that try to with switch_db(SomeDocument, alias):... on the same document in order to query multiple aliases.
Once in a while I get an exception:

File "/usr/local/lib/python2.7/dist-packages/mongoengine/queryset/queryset.py", line 104, in count
return super(QuerySet, self).count(with_limit_and_skip)
File "/usr/local/lib/python2.7/dist-packages/mongoengine/queryset/base.py", line 347, in count
return self._cursor.count(with_limit_and_skip=with_limit_and_skip)
File "/usr/local/lib/python2.7/dist-packages/mongoengine/queryset/base.py", line 1481, in _cursor
self._cursor_obj = self._collection.find(self._query,
AttributeError: 'NoneType' object has no attribute 'find'
File "/usr/local/lib/python2.7/dist-packages/mongoengine/queryset/queryset.py", line 104, in count
return super(QuerySet, self).count(with_limit_and_skip)
File "/usr/local/lib/python2.7/dist-packages/mongoengine/queryset/base.py", line 347, in count
return self._cursor.count(with_limit_and_skip=with_limit_and_skip)
File "/usr/local/lib/python2.7/dist-packages/mongoengine/queryset/base.py", line 1481, in _cursor
self._cursor_obj = self._collection.find(self._query,
AttributeError: 'NoneType' object has no attribute 'find'

It happens possibly due to self.cls._collection = None in switch_db.enter function.
I'm using mongoengine 0.10.6

Start with switch_db.enter and the _cursor path in mongoengine/queryset/base.py, especially where self.cls._collection becomes None. Reproduce the failure with concurrent green threads querying multiple aliases, then verify that concurrent switch_db use no longer produces a NoneType.find error.