Why does scheduleApplyErrata API fail with error message "Could not find method: scheduleApplyErrata" ?

Issue

• scheduleApplyErrata API fails with :

>>> client.system.scheduleApplyErrata(key, 1000011192)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.7/xmlrpclib.py", line 1224, in __call__
    return self.__send(self.__name, args)
  File "/usr/lib/python2.7/xmlrpclib.py", line 1578, in __request
    verbose=self.__verbose
  File "/usr/lib/python2.7/xmlrpclib.py", line 1264, in request
    return self.single_request(host, handler, request_body, verbose)
  File "/usr/lib/python2.7/xmlrpclib.py", line 1297, in single_request
    return self.parse_response(response)
  File "/usr/lib/python2.7/xmlrpclib.py", line 1473, in parse_response
    return u.close()
  File "/usr/lib/python2.7/xmlrpclib.py", line 793, in close
    raise Fault(**self._stack[0])
xmlrpclib.Fault: <Fault -1: 'redstone.xmlrpc.XmlRpcFault: Could not find method: scheduleApplyErrata in class: com.redhat.rhn.frontend.xmlrpc.system.SystemHandler with params: [java.lang.String, java.lang.Integer]'>