Why can't subscribe system to RHN option channel

Issue

# rhn-channel -a -c rhel-`uname -i`-server-optional-6 -u <RHN ID> -p <PASSWORD>

Traceback (most recent call last):
  File "/usr/sbin/rhn-channel", line 157, in <module>
    main()
  File "/usr/sbin/rhn-channel", line 121, in main
    result = subscribeChannels(OPTIONS.channel, OPTIONS.user, OPTIONS.password)
  File "/usr/share/rhn/up2date_client/rhnChannel.py", line 128, in subscribeChannels
    passwd)
  File "/usr/share/rhn/up2date_client/rhnserver.py", line 50, in __call__
    return rpcServer.doCall(method, *args, **kwargs)
  File "/usr/share/rhn/up2date_client/rpcServer.py", line 206, in doCall
    ret = method(*args, **kwargs)
  File "/usr/lib64/python2.6/xmlrpclib.py", line 1199, in __call__
    return self.__send(self.__name, args)
  File "/usr/share/rhn/up2date_client/rpcServer.py", line 39, in _request1
    ret = self._request(methodname, params)
  File "/usr/lib/python2.6/site-packages/rhn/rpclib.py", line 379, in _request
    request = self._req_body(self._strip_characters(params), methodname)
  File "/usr/lib/python2.6/site-packages/rhn/rpclib.py", line 234, in _req_body
    return xmlrpclib.dumps(params, methodname, encoding=self._encoding)
  File "/usr/lib64/python2.6/xmlrpclib.py", line 1132, in dumps
    data = m.dumps(params)
  File "/usr/lib64/python2.6/xmlrpclib.py", line 677, in dumps
    dump(v, write)
  File "/usr/lib64/python2.6/xmlrpclib.py", line 699, in __dump
    f(self, value, write)
  File "/usr/lib64/python2.6/xmlrpclib.py", line 703, in dump_nil
    raise TypeError, "cannot marshal None unless allow_none is enabled"
TypeError: cannot marshal None unless allow_none is enabled