addUserToGroup breaks in Python: No deserializer defined

The following Python code doesn't work:

user = soap.getUser(auth, "soapy")
print "Found user", user
group = soap.getGroup(auth, "jira-developers")
print "Found group", group
soap.addUserToGroup(auth, group, user)
print "Added to group"

It dies with:

<Fault soapenv:Server.userException: org.xml.sax.SAXException: No deserializer defined for array type

{http://soapinterop.org/xsd}

SOAPStruct: <SOAPpy.Types.structType detail at 1085258540>: {'hostname': 'atlassian01.contegix.com', 'faultData': <SOAPpy.Types.structType faultData at 1084456684>: {'exception': None, 'message': 'No deserializer defined for array type

{http://soapinterop.org/xsd}

SOAPStruct'}}>
Traceback (most recent call last):
File "./soapclient.py", line 58, in ?
soap.addUserToGroup(auth, group, user)
File "/usr/lib/python2.3/site-packages/SOAPpy/Client.py", line 421, in _call_
return self.__r_call(*args, **kw)
File "/usr/lib/python2.3/site-packages/SOAPpy/Client.py", line 443, in __r_call
self._hd, self._ma)
File "/usr/lib/python2.3/site-packages/SOAPpy/Client.py", line 357, in __call
raise p
SOAPpy.Types.faultType: <Fault soapenv:Server.userException: org.xml.sax.SAXException: No deserializer defined for array type

{http://soapinterop.org/xsd}

SOAPStruct: <SOAPpy.Types.structType detail at 1085258540>: {'hostname': 'atlassian01.contegix.com', 'faultData': <SOAPpy.Types.structType faultData at 1084456684>: {'exception': None, 'message': 'No deserializer defined for array type

{http://soapinterop.org/xsd}

SOAPStruct'}}>

The corresponding Java code does work.