Eventlet throwing unintelligble exceptions

Question

I'm using Eventlet's example code found here: http://eventlet.net/doc/examples.html#producer-consumer-recursive-web-crawler

Halfway through the run, I encounter multiple instances of the following:

Traceback (most recent call last):
  File "/usr/local/lib/python2.6/dist-packages/eventlet-0.9.9-py2.6.egg/eventlet/hubs/hub.py", line 285, in fire_timers
    timer()
  File "/usr/local/lib/python2.6/dist-packages/eventlet-0.9.9-py2.6.egg/eventlet/hubs/timer.py", line 56, in __call__
    cb(*args, **kw)
TypeError: exceptions must be classes, or instances, not bool