In Twister-core usernameCrawlwer.py failed at run-tme.

[Amit Kumar Yadav]

Hi All,
I am trying to run usernameCrawler.py file of twister-core. When i run this command:
sudo python usernameCrawler.py
it show thw error:
Traceback (most recent call last):
  File "/home/amit-b3/Amit_Repo/Project/twister-core/contrib/usernameCrawler.py", line 49, in <module>
    block = twister.getblock(nextHash)
  File "/usr/local/lib/python2.7/dist-packages/bitcoinrpc/authproxy.py", line 142, in __call__
    raise JSONRPCException(response['error'])
bitcoinrpc.authproxy.JSONRPCException: -1: value is type null, expected str

Please tell me i can execute it .