diff --git a/oauth3.implicit.js b/oauth3.implicit.js
index 4c27c4d..f1be21f 100644
--- a/oauth3.implicit.js
+++ b/oauth3.implicit.js
@@ -16,6 +16,8 @@
     }
   , _base64: {
       atob: function (base64) {
+        // atob must be called from the global context
+        // http://stackoverflow.com/questions/9677985/uncaught-typeerror-illegal-invocation-in-chrome
         return (exports.atob || require('atob'))(base64);
       }
     , decodeUrlSafe: function (b64) {
diff --git a/oauth3.issuer.mock.js b/oauth3.issuer.mock.js
index 24693de..b618ba0 100644
--- a/oauth3.issuer.mock.js
+++ b/oauth3.issuer.mock.js
@@ -3,7 +3,10 @@
 
   var OAUTH3 = exports.OAUTH3 = exports.OAUTH3 || require('./oauth3.implicit.js').OAUTH3;
 
-  OAUTH3._base64.btoa = exports.btoa || require('btoa');
+  OAUTH3._base64.btoa = function (b64) {
+    // http://stackoverflow.com/questions/9677985/uncaught-typeerror-illegal-invocation-in-chrome
+    (exports.btoa || require('btoa'))(b64);
+  }
   OAUTH3._base64.encodeUrlSafe = function (b64) {
     // Base64 to URL-safe Base64
     b64 = b64.replace(/\+/g, '-').replace(/\//g, '_');