Below is the exception that gets raised:

Exception in thread ReplicaSetMonitorThread (most likely raised during interpreter shutdown):
Traceback (most recent call last):
File "/usr/lib/python2.6/threading.py", line 532, in __bootstrap_inner
File "/usr/local/lib/python2.6/dist-packages/pymongo/replica_set_connection.py", line 109, in run
<type 'exceptions.AttributeError'>: 'NoneType' object has no attribute 'sleep'

Here is the patch that I did in my fork to fix this issue. https://github.com/chartbeat/mongo-python-driver/commit/a3ee17cadc6811538fdda5c3b8c9942a1a25d2bd