为什么xmlrpc客户端无法通过xmlrpc服务器过程将项目附加到列表中？

from xmlrpc.server import SimpleXMLRPCServer
from xmlrpc.server import SimpleXMLRPCRequestHandler

class RequestHandler(SimpleXMLRPCRequestHandler):
    rpc_paths = ()

server = SimpleXMLRPCServer(("127.0.0.1", 8000),
                            requestHandler=RequestHandler)
server.register_introspection_functions()

l = list()

def say_hi():
    return 'hi !'

def append(event):
    l.append(event)

server.register_function(say_hi)
server.register_function(append)

server.serve_forever()

>>> from xmlrpc.client import ServerProxy
>>> s = ServerProxy('http://127.0.0.1', allow_none=True)
>>> s.say_hi()
'hi !'
>>> s.append(1)
Traceback (most recent call last):
...
xmlrpc.client.Fault(Fault 1: "<class 'TypeError'>:cannot
                    marshal None unless allow_none is enabled")