spyne как отправить и получить xml

from spyne.application import Application 
from spyne.decorator import  rpc
from spyne.service import ServiceBase 
from spyne.model import String, Integer, xml, ComplexModel 
from spyne.protocol.soap import Soap11
from spyne.server.wsgi import WsgiApplication 
 
class MBPService(ServiceBase):
    @rpc(xml, _returns=xml) # определяет тип и количество принимаемых объектов и тип возвращаемого значения
    def command(ctx, arg1):
        print '========='
        print arg1
        print '========='
        return arg1
 
application = Application([MBPService], 'localhost',
                          in_protocol=Soap11(validator='lxml'),
                          out_protocol=Soap11())
 
wsgi_application = WsgiApplication(application)
 
if __name__ == '__main__':
    # далее поднимается наш сервер
    from wsgiref.simple_server import make_server
    print 'start server'
    server = make_server('127.0.0.1', 6363, wsgi_application)
    server.serve_forever()

Traceback (most recent call last):
  File "/home/user/test2/server.py", line 12, in <module>
    class MBPService(ServiceBase):
  File "/home/user/test2/spyne/service.py", line 53, in __init__
    descriptor = v(_default_function_name=k)
  File "/home/user/test2/spyne/decorator.py", line 341, in explain_method
    _no_ctx, _no_self, _args, body_style_str)
  File "/home/user/test2/spyne/decorator.py", line 115, in _produce_input_message
    namespace=ns, members=in_params)
  File "/home/user/test2/spyne/model/complex.py", line 1092, in produce
    '_type_info': TypeInfo(members),
  File "/home/user/test2/spyne/util/meta.py", line 66, in preparing_constructor
    return constructor(cls, name, bases, attributes)
  File "/home/user/test2/spyne/model/complex.py", line 587, in __new__
    _sanitize_type_info(cls_name, _type_info, _type_info_alt)
  File "/home/user/test2/spyne/model/complex.py", line 490, in _sanitize_type_info
    raise ValueError( (cls_name, k, v) )
ValueError: ('command', 'arg1', None)