Validation fails with TypeError

Given an individual PEM-encoded certificate cert, I want to validate it against a certification chain located in ./cc_certs/PT.pem which I open to build a list of intermediate certificates:

def validate(cert):
    intermediates = []
    with open('./cc_certs/PT.pem', 'rb') as f:
        for _, _, der_bytes in pem.unarmor(f.read(), multiple=True):
            intermediates.append(der_bytes)

    validator = CertificateValidator(cert, intermediates)
    validator.validate_usage(set(['digital_signature']))

Simple use-case that's well documented. However when running this it blows up complaining about a TypeError

Traceback (most recent call last):
  File "client.py", line 114, in <module>
    smartcard.validate(cc_cert)
  File "/Users/fabio/Code/security2017-p1g2/src/chat/smartcard.py", line 38, in validate
    validator.validate_usage(set(['digital_signature']))
  File "/usr/local/lib/python3.6/site-packages/certvalidator/__init__.py", line 193, in validate_usage
    self._validate_path()
  File "/usr/local/lib/python3.6/site-packages/certvalidator/__init__.py", line 121, in _validate_path
    validate_path(self._context, candidate_path)
  File "/usr/local/lib/python3.6/site-packages/certvalidator/validate.py", line 50, in validate_path
    return _validate_path(validation_context, path)
  File "/usr/local/lib/python3.6/site-packages/certvalidator/validate.py", line 407, in _validate_path
    end_entity_name_override=end_entity_name_override
  File "/usr/local/lib/python3.6/site-packages/certvalidator/validate.py", line 1243, in verify_crl
    if isinstance(distribution_point['crl_issuer'], x509.GeneralNames):
TypeError: string indices must be integers

Sometimes the error also happens to be

Traceback (most recent call last):
  File "client.py", line 114, in <module>
    smartcard.validate(cc_cert)
  File "/Users/fabio/Code/security2017-p1g2/src/chat/smartcard.py", line 39, in validate
    validator.validate_usage(set(['digital_signature']))
  File "/usr/local/lib/python3.6/site-packages/certvalidator/__init__.py", line 193, in validate_usage
    self._validate_path()
  File "/usr/local/lib/python3.6/site-packages/certvalidator/__init__.py", line 121, in _validate_path
    validate_path(self._context, candidate_path)
  File "/usr/local/lib/python3.6/site-packages/certvalidator/validate.py", line 50, in validate_path
    return _validate_path(validation_context, path)
  File "/usr/local/lib/python3.6/site-packages/certvalidator/validate.py", line 386, in _validate_path
    end_entity_name_override=end_entity_name_override
  File "/usr/local/lib/python3.6/site-packages/certvalidator/validate.py", line 956, in verify_ocsp_response
    if moment > cert_response['next_update'].native:
TypeError: '>' not supported between instances of 'datetime.datetime' and 'NoneType'

What do these errors mean? Or why does the error appear to be random? I tried to trace the exception in the source code but I don't know why it's being raised.

Start with the validation call in client.py and smartcard.py, then inspect certvalidator/validate.py at verify_crl() and verify_ocsp_response(), the traceback locations named in the report. Reproduce the provided certificate-chain validation flow and examine the differing CRL and OCSP data that reaches those checks. Done means identifying why the input types or missing values vary and documenting or correcting the resulting validation behavior.