Document FTP server does not understand "cd .." parent directory change

The document_ftp does not understand a "cd .." command, as issued by most, if not all, FTP clients for accessing a parent directory.

ftp> open servername 8021
connection established
220 pyftpdlib 0.4.0 ready.
user (servername:(none)): admin
331 Username ok, send password.
Password:
230 Welcome on OpenERP document management system.
ftp> ls
200 Active data connection established.
125 Data connection already open. Transfer starting.
test_ftp1
test_ftp2
226 Transfer complete.
FTP: 89 Bytes received in 0,00seconds 89,00KB/s
ftp> cd test_ftp1
250 "/test_ftp1/" is the current directory.
ftp> ls
200 Active data connection established.
125 Data connection already open. Transfer starting.
Documents
226 Transfer complete.
FTP: 11 Bytes empfangen in 0,00Sekunden 11000,00KB/s
ftp> cd ..
451 Exception.

[2011-03-14 16:23:58,416][test_ftp1] ERROR:FTP:[01]: Traceback (most recent call last):
[2011-03-14 16:23:58,416][test_ftp1] ERROR:FTP:[02]: File "/usr/local/lib/python2.6/dist-packages/openerp-server/addons/document_ftp/ftpserver/ftpserver.py", line 1800, in try_as_current_user
[2011-03-14 16:23:58,416][test_ftp1] ERROR:FTP:[03]: return self.run_as_current_user(function, *args, **kwargs)
[2011-03-14 16:23:58,416][test_ftp1] ERROR:FTP:[04]: File "/usr/local/lib/python2.6/dist-packages/openerp-server/addons/document_ftp/ftpserver/ftpserver.py", line 1775, in run_as_current_user
[2011-03-14 16:23:58,417][test_ftp1] ERROR:FTP:[05]: return function(*args, **kwargs)
[2011-03-14 16:23:58,417][test_ftp1] ERROR:FTP:[06]: File "/usr/local/lib/python2.6/dist-packages/openerp-server/addons/document_ftp/ftpserver/abstracted_fs.py", line 276, in get_crdata
[2011-03-14 16:23:58,417][test_ftp1] ERROR:FTP:[07]: assert '..' not in p_parts
[2011-03-14 16:23:58,417][test_ftp1] ERROR:FTP:[08]: AssertionError
[2011-03-14 16:23:58,417][test_ftp1] INFO:FTP:[admin]@192.168.1.123:50178 FAIL get_crdata() ".." Exception: Exception.