“X-Accel-Mapping header missing” in Nginx error log

Question

In Rails in my controller I set response header explicitly :

   response.headers['X-Accel-Redirect'] = "some_url"

Nginx Config

   location ~* ^/some_url/(.*){     

            set $s3_bucket        '$arg_bucket_name';
            set $aws_access_key   'AWSAccessKeyId=$arg_AWSAccessKeyId';
            set $url_expires      'Expires=$arg_Expires';
            set $url_signature    'Signature=$arg_Signature';
            set $url_full         '$1?$aws_access_key&$url_expires&$url_signature';
            proxy_hide_header Content-Disposition;
            add_header Content-Disposition 'attachment; filename=$arg_filename';
            proxy_hide_header      x-amz-id-2;
            proxy_hide_header      x-amz-request-id;
            proxy_buffering        off;
            proxy_intercept_errors on;
            resolver               4.2.2.2 8.8.8.8 valid=300s;
            proxy_pass             https://$s3_bucket/$url_full;
} 

What is the reason for this and how I can get rid of this?