public function assertResponseHeaderDoesNotExist($header) {
        $this->requireResponse();

        try {
            Assertion::false(
                $this->response->hasHeader($header),
                sprintf('The "%s" response header should not exist.', $header)
            );
        } catch (AssertionFailure $e) {
            throw new AssertionFailedException($e->getMessage());
        }
    }