No authentication prompt with if block

[Grant]

Authentication works fine if I don't include the if block but I'd like
to allow only a certain user access to this server block.  I get a 403
in the browser without any prompt for authentication.

auth_basic "Authentication Required";
auth_basic_user_file htpasswd;
if ($remote_user != "myuser") {
    return 403;
}

What am I doing wrong?

- Grant