str is disallowed in the filter clause.

Raise code

ck_filterable(self, expression):
        """Raise an error if expression cannot be used in a WHERE clause."""
        if (
            hasattr(expression, 'resolve_expression') and
            not getattr(expression, 'filterable', True)
        ):
            raise NotSupportedError(
                expression.__class__.__name__ + ' is disallowed in the filter '
                'clause.'
            )
        if hasattr(expression, 'get_source_expressions'):
            for expr in expression.get_source_expressions():
                self.check_filterable(expr)

    def buil