WSDL module: TypeError: cannot concatenate 'str' and 'NoneType' objects

Question

I'm trying to use the WSDL package in python. The error I'm getting is associated with other packages and I'm not sure how to trace the problem. Here's the error:

python flightaware.py
http://flightxml.flightaware.com/soap/FlightXML2
__main__.myHTTPTransport

Traceback (most recent call last):
File "flightaware.py", line 37, in <module>
  DF = WSDL.Proxy(wsdlFile, transport=myHTTPTransport)
File "/usr/local/lib/python2.7/distpackages/SOAPpy0.12.22py2.7.egg/SOAPpy/WSDL.py", line 74, in __init__
  stream = urllib.URLopener(key_file=config.SSL.key_file, cert_file=config.SSL.cert_file).open(wsdlsource)
File "/usr/lib/python2.7/urllib.py", line 203, in open
  name = 'open_' + urltype
TypeError: cannot concatenate 'str' and 'NoneType' objects

Any ideas would help.