我试图写一个函数来终止我的程序。它叫做quitChat。但是当我调用函数messenger:quitChat()时，我得到了这个错误。

** exception error: bad argument
    in function  exit/2
        called as exit(undefined,kill)
    in call from messenger:quitChat/0 (messenger.erl, line 26)

到目前为止，我使用的是以下代码：

-module(messenger).
-export([start_server/0, server/1, logon/1, logoff/0, message/2, client/2,
requestChat/1, quitChat/0]).

server_node() ->
    messenger@timberlake.

server(User_List) ->
    receive
        {From, logon, Name} ->
            New_User_List = server_logon(From, Name, User_List),
            server(New_User_List);
        {From, logoff} ->
            New_User_List = server_logoff(From, User_List),
            server(New_User_List);
        {From, message_to, To, Message} ->
            server_transfer(From, To, Message, User_List),
            io:format("list is now: ~p~n", [User_List]),
            server(User_List)
    end.

start_server() ->
    register(messenger, spawn(messenger, server, [[]])).

quitChat() ->
    exit(whereis(messenger), kill).