cant backup

When ever I try to do a backup it gets about 2/3 the way through then I get the following error:

Traceback (most recent call last):
  File "/usr/bin/duplicity", line 1412, in <module>
    with_tempdir(main)
  File "/usr/bin/duplicity", line 1407, in with_tempdir
    tempdir.default().cleanup()
  File "/usr/lib/python2.7/dist-packages/duplicity/tempdir.py", line 58, in default
    _defaultInstance = TemporaryDirectory(temproot = globals.temproot)
  File "/usr/lib/python2.7/dist-packages/duplicity/tempdir.py", line 117, in __init__
    self.__dir = tempfile.mkdtemp("-tempdir", "duplicity-", temproot)
  File "/usr/lib/python2.7/tempfile.py", line 317, in mkdtemp
    dir = gettempdir()
  File "/usr/lib/python2.7/tempfile.py", line 261, in gettempdir
    tempdir = _get_default_tempdir()
  File "/usr/lib/python2.7/tempfile.py", line 208, in _get_default_tempdir
    ("No usable temporary directory found in %s" % dirlist))
IOError: [Errno 2] No usable temporary directory found in ['/tmp', '/var/tmp', '/usr/tmp', '/home/digital']