def _check_if_states_are_strings(cls):
        """Check if all states are strings."""
        for item in list(cls.context.states_enum):
            if not isinstance(item.value, six.string_types):
                raise ValueError(
                    'Item {name} is not string. Only strings are allowed.'
                    .format(name=item.name)
                )
Check if all states are strings.