docs: def validate_unwrap(self, value):
          ''' Checks that value is a ``dict``, that every key is a valid MongoDB
              key, and that every value validates based on DictField.value_type
          '''
          if not isinstance(value, dict):
              self._fail_validation_type(value, dict)
          for k, v in value.items():
              self._validate_key_unwrap(k)
              try:
                  self.value_type.validate_unwrap(v)
              except BadValueException as bve:
                  self._fail_validation(value, 'Bad value for key %s' % k, cause=bve)