ImportError import name import_string

[Priyadharshini S]

when i am trying to install ratticDB on ubuntu 12.04 i got this below error
 Please help me to rectify it
./manage.py syncdb --noinput
Traceback (most recent call last):
File "./manage.py", line 10, in
execute_from_command_line(sys.argv)
File "/usr/local/lib/python2.7/dist-packages/django/core/management/init.py", line 399, in execute_from_command_line
utility.execute()
File "/usr/local/lib/python2.7/dist-packages/django/core/management/init.py", line 392, in execute
self.fetch_command(subcommand).run_from_argv(self.argv)
File "/usr/local/lib/python2.7/dist-packages/django/core/management/base.py", line 242, in run_from_argv
self.execute(args, *options.dict)
File "/usr/local/lib/python2.7/dist-packages/django/core/management/base.py", line 284, in execute
self.validate()
File "/usr/local/lib/python2.7/dist-packages/django/core/management/base.py", line 310, in validate
num_errors = get_validation_errors(s, app)
File "/usr/local/lib/python2.7/dist-packages/django/core/management/validation.py", line 34, in get_validation_errors
for (app_name, error) in get_app_errors().items():
File "/usr/local/lib/python2.7/dist-packages/django/db/models/loading.py", line 196, in get_app_errors
self.populate()
File "/usr/local/lib/python2.7/dist-packages/django/db/models/loading.py", line 78, in populate
self.load_app(app_name)
File "/usr/local/lib/python2.7/dist-packages/django/db/models/loading.py", line 99, in load_app
models = import_module('%s.models' % app_name)
File "/usr/local/lib/python2.7/dist-packages/django/utils/importlib.py", line 40, in import_module
__import(name)
File "/usr/local/lib/python2.7/dist-packages/two_factor/models.py", line 20, in
from .gateways import make_call, send_sms
File "/usr/local/lib/python2.7/dist-packages/two_factor/gateways/init.py", line 2, in
from django.utils.module_loading import import_string
ImportError: cannot import name import_string

[Priyadharshini S]

please help me to solve this