diff --git a/package.json b/package.json
index 40bc320..d7003db 100644
--- a/package.json
+++ b/package.json
@@ -19,7 +19,7 @@
   "module": "dist/idiomorph.esm.js",
   "unpkg": "dist/idiomorph.min.js",
   "scripts": {
-    "test": "mocha-chrome test/index.html",
+    "test": "mocha-chrome test/index.html --chrome-flags '[\"--allow-file-access-from-files\"]'",
     "amd": "(echo \"define(() => {\n\" && cat src/idiomorph.js && echo \"\nreturn Idiomorph});\") > dist/idiomorph.amd.js",
     "cjs": "(cat src/idiomorph.js && echo \"\nmodule.exports = Idiomorph;\") > dist/idiomorph.cjs.js",
     "esm": "(cat src/idiomorph.js && echo \"\nexport {Idiomorph};\") > dist/idiomorph.esm.js",
diff --git a/src/idiomorph.js b/src/idiomorph.js
index a142bd5..1ac7b6d 100644
--- a/src/idiomorph.js
+++ b/src/idiomorph.js
@@ -672,10 +672,10 @@ var Idiomorph = (function () {
             Object.assign(finalConfig, config);
 
             // copy callbacks into final config (do this to deep merge the callbacks)
-            Object.assign(finalConfig.callbacks, config.callbacks);
+            finalConfig.callbacks = Object.assign({}, defaults.callbacks, config.callbacks);
 
             // copy head config into final config  (do this to deep merge the head)
-            Object.assign(finalConfig.head, config.head);
+            finalConfig.head = Object.assign({}, defaults.head, config.head);
 
             return finalConfig;
         }