KeyError: 'message' in using signing service for signing a transferMsg

[Mahsa-Bastankhah]

Hi. I'm running the following code :
signing_client = HttpApiSigningClient('http://127.0.0.1:8000', username='sam', password='mypass')
client = HttpApiClient(env=testnet_env)
transfer_msg = TransferMsg(
symbol='BNB',
amount=1,
to_address='tbnb15lq6x48g3pvaqjwhqfg4nygx63q03zsvk9rlxs')
signed_msg = signing_client.sign_transfer(transfer_msg, wallet_name='wallet_3')
res = signing_client.broadcast_transfer(signed_msg, wallet_name="wallet_3")

and I get this error raised from signed_msg = signing_client.sign_transfer(transfer_msg, wallet_name='wallet_3') :

"File "sendBNB.py", line 23, in
signed_msg = signing_client.sign_transfer(transfer_msg, wallet_name='wallet_3')
File "/home/mahsa/.local/lib/python3.6/site-packages/binance_chain/signing/http.py", line 266, in sign_transfer
return self._post('transfer/sign', json=data)
File "/home/mahsa/.local/lib/python3.6/site-packages/binance_chain/signing/http.py", line 122, in _post
return self._request('post', path, **kwargs)
File "/home/mahsa/.local/lib/python3.6/site-packages/binance_chain/signing/http.py", line 93, in _request
return self._handle_response(response)
File "/home/mahsa/.local/lib/python3.6/site-packages/binance_chain/signing/http.py", line 104, in _handle_response
raise BinanceChainAPIException(response, response.status_code)
File "/home/mahsa/.local/lib/python3.6/site-packages/binance_chain/exceptions.py", line 17, in init
self.message = json_res['message']
KeyError: 'message'"

my uvicorn server also prints :
INFO: 127.0.0.1:46392 - "POST /api/auth/login HTTP/1.1" 200 OK
INFO: 127.0.0.1:46394 - "POST /api/auth/login HTTP/1.1" 200 OK
INFO: 127.0.0.1:46394 - "POST /api/transfer/broadcast HTTP/1.1" 422 Unprocessable Entity

the weird fact is that when I use signing service for NewOrderMsg it works without any problem!
what should I do?