Basic Auth not requested when rewrite rule exists

[Dave Kennard]

Part of my Server config looks like this:
location /admin/ {
         auth_basic            "Restricted";
         auth_basic_user_file  /path/to/.htpasswd;

         #rewrite any .xhtml page to page.php
         rewrite ^/(.+)\.xhtml$ /$1.php last;
     }

But when I go to mysite.com/admin/somefile.xhtml I don't get asked for 
Authentification and the page will display.

If I comment out the rewrite rule then I do get asked for authentification.

Can anyone advise me on how to get authentification and the rewrite rule 
working together?

Thanks

Dave
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