keystone-manage db_sync issues error traceback in the terminal

I did a fresh devstack install and everything is running fine. Now I logged in a terminal without any special environment variables set and issued the command $ keystone-manage db_sync

deepak@deepak-devvm:~$ keystone-manage db_sync
Traceback (most recent call last):
  File "/usr/local/bin/keystone-manage", line 7, in <module>
    execfile(__file__)
  File "/opt/stack/keystone/bin/keystone-manage", line 28, in <module>
    cli.main(argv=sys.argv, config_files=config_files)
  File "/opt/stack/keystone/keystone/cli.py", line 147, in main
    return run(cmd, (args[:1] + args[2:]))
  File "/opt/stack/keystone/keystone/cli.py", line 133, in run
    return CMDS[cmd](argv=args).run()
  File "/opt/stack/keystone/keystone/cli.py", line 35, in run
    return self.main()
  File "/opt/stack/keystone/keystone/cli.py", line 54, in main
    driver = utils.import_object(getattr(CONF, k).driver)
  File "/opt/stack/keystone/keystone/common/utils.py", line 57, in import_object
    __import__(import_str)
TypeError: __import__() argument 1 must be string, not None