Error when updating WIT field

I am able to get details of a wit field using print(workitem['state'] ) but i am getting error when updating it..

I am getting below error when updating wit field:

Traceback (most recent call last):
File "C:\Python35-32\lib\site-packages\dohq_tfs-1.0.dev0-py3.5.egg\tfs\connection.py", line 224, in __send_request
File "C:\Python35-32\lib\site-packages\requests-2.18.4-py3.5.egg\requests\models.py", line 892, in json
return complexjson.loads(self.text, **kwargs)
File "C:\Python35-32\lib\json\__init__.py", line 319, in loads
return _default_decoder.decode(s)
File "C:\Python35-32\lib\json\decoder.py", line 339, in decode
obj, end = self.raw_decode(s, idx=_w(s, 0).end())
File "C:\Python35-32\lib\json\decoder.py", line 357, in raw_decode
raise JSONDecodeError("Expecting value", s, err.value) from None
json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "D:/Release/PythonProjects/TFSAPI.py", line 33, in
workitem['state'] = 'Pending Review'
File "C:\Python35-32\lib\site-packages\dohq_tfs-1.0.dev0-py3.5.egg\tfs\resources.py", line 89, in __setitem__
File "C:\Python35-32\lib\site-packages\dohq_tfs-1.0.dev0-py3.5.egg\tfs\connection.py", line 107, in update_workitem
File "C:\Python35-32\lib\site-packages\dohq_tfs-1.0.dev0-py3.5.egg\tfs\connection.py", line 189, in send_patch
File "C:\Python35-32\lib\site-packages\dohq_tfs-1.0.dev0-py3.5.egg\tfs\connection.py", line 231, in __send_request
tfs.connection.TFSClientError: Response is not json: Invalid Http Request Header