Satellite activation key api

I have a script to change the channels in an activation key using the satellite api. It looks something similar to this:
clone_channel_details = {
'base_channel_label' : "TEST-rhel-x86_64-server-6-update-4-11192013"
}

client.activationkey.setDetails(key,one_key,clone_channel_details)

This changes the base channel. I can see the children channels but they aren't selected. So I thought the addChildChannels method would "select" the children channels for the activation key, but I get an error when I run this:

client.activationkey.addChildChannels(key,one_key,"TEST-rhel-x86_64-server-optional-6-update-4-11192013")

Here is the error:

Traceback (most recent call last):
File "./update_activation_key.py", line 21, in
client.activationkey.addChildChannels(key,one_key,"TEST-rhel-x86_64-server-optional-6-update-4-11192013")
File "/usr/lib64/python2.6/xmlrpclib.py", line 1199, in call
return self.__send(self.__name, args)
File "/usr/lib64/python2.6/xmlrpclib.py", line 1489, in __request
verbose=self.__verbose
File "/usr/lib64/python2.6/xmlrpclib.py", line 1253, in request
return self._parse_response(h.getfile(), sock)
File "/usr/lib64/python2.6/xmlrpclib.py", line 1392, in _parse_response
return u.close()
File "/usr/lib64/python2.6/xmlrpclib.py", line 838, in close
raise Fault(**self._stack[0])
xmlrpclib.Fault:

Does anyone know why I am getting this error? Am I using the correct method to assign the children channels?