diff --git a/index.js b/index.js
index 6a378083..6718031a 100644
--- a/index.js
+++ b/index.js
@@ -249,10 +249,18 @@ function normalizeAndroidVersions(androidVersions, chromeVersions) {
     .concat(chromeVersions.slice(iFirstEvergreen))
 }
 
+function copyObject(obj) {
+  var copy = {}
+  for (var key in obj) {
+    copy[key] = obj[key]
+  }
+  return copy
+}
+
 function normalizeAndroidData(android, chrome) {
   android.released = normalizeAndroidVersions(android.released, chrome.released)
   android.versions = normalizeAndroidVersions(android.versions, chrome.versions)
-  android.releaseDate = Object.assign({}, android.releaseDate)
+  android.releaseDate = copyObject(android.releaseDate)
   android.released.forEach(function (v) {
     if (android.releaseDate[v] === undefined) {
       android.releaseDate[v] = chrome.releaseDate[v]