If command seems not working? what is wrong with me?

Server NGINX
1

i have following config:

                  if ($http_user_agent ~* Opera ) {
                            return 444;
                    }

but, even if I try to access pages using Opera, Opera can open pages.
What is wrong with my config ?

(for Operat lovers: normally I do not want to block opera, just
testing something )

Thanks.