error in backup blobs directory

root@schooltool:~# repozo -B -v -r /home/admincek/Downloads/blobs -f /var/lib/schooltool/blobs
looking for files between last full backup and 2016-08-12-06-05-53...
no files found
doing a full backup
Traceback (most recent call last):
  File "/usr/bin/repozo", line 9, in <module>
    load_entry_point('ZODB3==3.10.5', 'console_scripts', 'repozo')()
  File "/usr/lib/python2.7/dist-packages/ZODB/scripts/repozo.py", line 611, in main
    do_backup(options)
  File "/usr/lib/python2.7/dist-packages/ZODB/scripts/repozo.py", line 500, in do_backup
    do_full_backup(options)
  File "/usr/lib/python2.7/dist-packages/ZODB/scripts/repozo.py", line 436, in do_full_backup
    fs = FileStorage(options.file, read_only=True)
  File "/usr/lib/python2.7/dist-packages/ZODB/FileStorage/FileStorage.py", line 148, in __init__
    self._file = open(file_name, read_only and 'rb' or 'r+b')
IOError: [Errno 21] Is a directory: '/var/lib/schooltool/blobs'