diff --git a/services/filestore/app/js/GcsPersistor.js b/services/filestore/app/js/GcsPersistor.js
index a78bfce2cd..5a44132882 100644
--- a/services/filestore/app/js/GcsPersistor.js
+++ b/services/filestore/app/js/GcsPersistor.js
@@ -113,7 +113,8 @@ async function sendStream(bucketName, key, readStream, sourceMd5) {
   }
 }
 
-async function getFileStream(bucketName, key, opts = {}) {
+async function getFileStream(bucketName, key, _opts = {}) {
+  const opts = Object.assign({}, _opts)
   if (opts.end) {
     // S3 (and http range headers) treat 'end' as inclusive, so increase this by 1
     opts.end++