looking for index.php while proxy_pass to s3 bucket

I am configuring my nginx proxy server to fetch data from s3 bucket in case of 502 error.

This is my configuration file.

error_page 502 @static;

location @static{
  rewrite ^/a/b /a_b break;
  proxy_pass_request_headers off;
  proxy_set_header Host bucket.s3.amazonaws.com;
  proxy_hide_header x-amz-id-2;
  proxy_hide_header x-amz-request-id;
  proxy_pass http://ift.tt/1pVsyYJ;
}

According to me, above code should bring data from http://ift.tt/1LHT9T0, but it isn't doing that. It is trying to access http://ift.tt/1LHT4ik, I don't know why? Can some one please explain that?