rhn-channel displayed error message when adding the channel

Issue

• After registerd the system, and then, executed following command to add channel.

 # rhn-channel --add --channel=rhel-x86_64-server-optional-6

• After executing the command and input RHN-ID/password, following error is displayed and failed to add the channel:

   <type 'exceptions.TypeError'>
  
   In /var/log/up2date, following error is logged:
   Traceback (most recent call last):
     File "/usr/sbin/rhn-channel", line 159, in <module>
       main()
     File "/usr/sbin/rhn-channel", line 122, 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 51, 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"
   <type 'exceptions.TypeError'>: cannot marshal None unless allow_none is enabled
  

  (1) Why this error is displayed?
  (2) How we add channel to the system correctly?