diff --git a/lib/adapters/xhr.js b/lib/adapters/xhr.js
index 1174a86..f5c397c 100644
--- a/lib/adapters/xhr.js
+++ b/lib/adapters/xhr.js
@@ -6,7 +6,7 @@ var buildURL = require('./../helpers/buildURL');
 var parseHeaders = require('./../helpers/parseHeaders');
 var isURLSameOrigin = require('./../helpers/isURLSameOrigin');
 var createError = require('../core/createError');
-var btoa = (typeof window !== 'undefined' && window.btoa) || require('./../helpers/btoa');
+var btoa = (typeof window !== 'undefined' && window.btoa && window.btoa.bind(window)) || require('./../helpers/btoa');
 
 module.exports = function xhrAdapter(config) {
   return new Promise(function dispatchXhrRequest(resolve, reject) {