[Zope] BUG? ftp download of GIF

[Li Dongfeng]

The problem:
Trying to ftp download one GIF image in a zope folder
failed, the server give the following error message:

------
1999-12-24T03:14:10 ERROR(200) ZServer exception in trigger thunk:
(exceptions.AttributeError:'None' object has no attribute '_producers'
[/home/maldf/Zope-2.1.0b2-src/ZServer/medusa/select_trigger.py|handle_read|82]
[/home/maldf/Zope-2.1.0b2-src/ZServer/FTPResponse.py|apply|145]
[/home/maldf/Zope-2.1.0b2-src/ZServer/FTPServer.py|retr_completion|360])

But many other GIF images never cause this
problem in ftp download.
The offending image is at
  http://www.zope.org/Members/LiDongfeng/test.gif
The ftp client used is WS FTP LE version 5.06 on windows.
The Zope server is version 2.1.0b2 on solaris 2.6.

Anyone know the cause of this problem?




PS:
I have found the offending code in
ZServer/FTPServer.py|retr_completion|360, 
it is:

            if not response._wrote:
                self.client_dc.push(response.body)
            else:
                for producer in response.stdout._producers:
                    self.client_dc.push_with_producer(producer)

obviously the response.stdout is None.
Why this occured?