code: def _check_audience(payload_dict, audience):
      
      if audience is None:
          return
  
      audience_in_payload = payload_dict.get('aud')
      if audience_in_payload is None:
          raise AppIdentityError(
              'No aud field in token: {0}'.format(payload_dict))
      if audience_in_payload != audience:
          raise AppIdentityError('Wrong recipient, {0} != {1}: {2}'.format(
              audience_in_payload, audience, payload_dict))
docs: Checks audience field from a JWT payload.
  
      Does nothing if the passed in ``audience`` is null.
  
      Args:
          payload_dict: dict, A dictionary containing a JWT payload.
          audience: string or NoneType, an audience to check for in
                    the JWT payload.
  
      Raises:
          AppIdentityError: If there is no ``'aud'`` field in the payload
                            dictionary but there is an ``audience`` to check.
          AppIdentityError: If the ``'aud'`` field in the payload dictionary
                            does not match the ``audience``.