Negative salt causes a crash

[paulie4]

I've occasionally seen Cognito return a negative hex number for the ChallengeParameters.SALT value, which causes warrant to crash:

Traceback (most recent call last):
  File "cogtest.py", line 5, in <module>
    cog.authenticate('password')
  File "C:\Program Files\Python38\lib\site-packages\warrant\__init__.py", line 382, in authenticate
    tokens = aws.authenticate_user()
  File "C:\Program Files\Python38\lib\site-packages\warrant\aws_srp.py", line 209, in authenticate_user
    challenge_response = self.process_challenge(response['ChallengeParameters'])
  File "C:\Program Files\Python38\lib\site-packages\warrant\aws_srp.py", line 183, in process_challenge
    hkdf = self.get_password_authentication_key(user_id_for_srp,
  File "C:\Program Files\Python38\lib\site-packages\warrant\aws_srp.py", line 152, in get_password_authentication_key
    x_value = hex_to_long(hex_hash(pad_hex(salt) + username_password_hash))
  File "C:\Program Files\Python38\lib\site-packages\warrant\aws_srp.py", line 35, in hex_hash
    return hash_sha256(bytearray.fromhex(hex_string))
ValueError: non-hexadecimal number found in fromhex() arg at position 1

Is there documentation somewhere that you used to create this code, and does it specify what the valid values of the salt can be?