AttributeError: 'NoneType' object has no attribute 'startswith'

From the mail list:

I did several tests with duplicity 0.6.0 and got an error with every try. Different form another post here it's independant of a pending slash always the same error:

~$ /usr/local/bin/duplicity --encrypt-key 6C2B1FE5 --sign-key 6C2B1FE5 --verbosity 9 /home/karl/Dokumente/nbook-dok/ ftp://<email address hidden>/ba_test/
NcFTP version is 3.2.1
Password for 'domain.de':
Using temporary directory /tmp/duplicity-KKpF9W-tempdir
Registering (mkstemp) temporary file /tmp/duplicity-KKpF9W-tempdir/mkstemp-MdchAV-1
Main action: inc
================================================================================
duplicity 0.6.00 (June 08, 2009)
Args: /usr/local/bin/duplicity --encrypt-key 6C2B1FE5 --sign-key 6C2B1FE5 --verbosity 9 /home/karl/Dokumente/nbook-dok/ ftp://<email address hidden>/ba_test/
Linux nbook 2.6.28-13-generic #44-Ubuntu SMP Tue Jun 2 07:57:31 UTC 2009 i686
/usr/bin/python 2.6.2 (release26-maint, Apr 19 2009, 01:56:41)
[GCC 4.3.3]
================================================================================
Registering (mkstemp) temporary file /tmp/duplicity-KKpF9W-tempdir/mkstemp-ildtEB-2
Temp has 15484612608 available, backup will use approx 34078720.
Reading results of 'ncftpls -f /tmp/duplicity-KKpF9W-tempdir/mkstemp-MdchAV-1 -F -t 30 -o useCLNT=0,useHELP_SITE=0 -l 'ftp://domain.de/ba_test/''
2 files exist on backend
0 files exist in cache
Extracting backup chains from list of files: ['.', '..']
File . is not part of a known set; creating new set
Ignoring file (rejected by backup set) '.'
File .. is not part of a known set; creating new set
Ignoring file (rejected by backup set) '..'
Last full backup date: none
Collection Status
---
...
Connecting with backend: FTPBackend
Archive dir: (() /home/karl/.duplicity dir)

Found 0 backup chains without signatures.
No backup chains with active signatures found
No orphaned or incompletme backup sets found.
PASSPHRASE variable not set, asking user.
GnuPG passphrase:
....
Forgetting temporary file /tmp/duplicity-KKpF9W-tempdir/mktemp-CUE7Dg-3
AsyncScheduler: task completed successfully
Processed volume 1
Removing still remembered temporary file /tmp/duplicity-KKpF9W-tempdir/mkstemp-ildtEB-2
Removing still remembered temporary file /tmp/duplicity-KKpF9W-tempdir/mkstemp-MdchAV-1
Traceback (most recent call last):
 File "/usr/local/bin/duplicity", line 1002, in <module>
   with_tempdir(main)
 File "/usr/local/bin/duplicity", line 995, in with_tempdir
   fn()
 File "/usr/local/bin/duplicity", line 974, in main
   full_backup(col_stats)
 File "/usr/local/bin/duplicity", line 415, in full_backup
   sig_outfp.to_remote()
 File "/usr/local/lib/python2.6/dist-packages/duplicity/dup_temp.py", line 186, in to_remote
   globals.backend.put(tgt)
 File "/usr/local/lib/python2.6/dist-packages/duplicity/backends/ftpbackend.py", line 90, in put
   remote_path = os.path.join(urllib.unquote(self.parsed_url.path.lstrip('/')), remote_filename).rstrip()
 File "/usr/lib/python2.6/posixpath.py", line 65, in join
   if b.startswith('/'):
AttributeError: 'NoneType' object has no attribute 'startswith'