diff --git a/fetch.js b/fetch.js
index f89f27a6..c8b19242 100644
--- a/fetch.js
+++ b/fetch.js
@@ -1,7 +1,7 @@
 (function() {
   'use strict';
 
-  if (self.fetch) {
+  if (typeof window !== 'object' || window.fetch) {
     return
   }
 
@@ -108,7 +108,7 @@
   }
 
   var support = {
-    blob: 'FileReader' in self && 'Blob' in self && (function() {
+    blob: 'FileReader' in window && 'Blob' in window && (function() {
       try {
         new Blob();
         return true
@@ -116,8 +116,8 @@
         return false
       }
     })(),
-    formData: 'FormData' in self,
-    arrayBuffer: 'ArrayBuffer' in self
+    formData: 'FormData' in window,
+    arrayBuffer: 'ArrayBuffer' in window
   }
 
   function Body() {
@@ -312,11 +312,11 @@
     return new Response(null, {status: status, headers: {location: url}})
   }
 
-  self.Headers = Headers;
-  self.Request = Request;
-  self.Response = Response;
+  window.Headers = Headers;
+  window.Request = Request;
+  window.Response = Response;
 
-  self.fetch = function(input, init) {
+  window.fetch = function(input, init) {
     return new Promise(function(resolve, reject) {
       var request
       if (Request.prototype.isPrototypeOf(input) && !init) {
@@ -377,5 +377,5 @@
       xhr.send(typeof request._bodyInit === 'undefined' ? null : request._bodyInit)
     })
   }
-  self.fetch.polyfill = true
+  window.fetch.polyfill = true
 })();
diff --git a/test/test-worker.html b/test/test-worker.html
index 6ff9dd52..2f319c1a 100644
--- a/test/test-worker.html
+++ b/test/test-worker.html
@@ -30,7 +30,7 @@
           })
           break
         case 'end':
-          if (self.mochaPhantomJS) {
+          if (window.mochaPhantomJS) {
             mochaPhantomJS.run()
           } else {
             mocha.run()
diff --git a/test/test.html b/test/test.html
index 8047ec51..9f6fbefb 100644
--- a/test/test.html
+++ b/test/test.html
@@ -11,7 +11,7 @@
   <script src="../node_modules/mocha/mocha.js"></script>
   <script>
     mocha.setup('tdd')
-    self.assert = chai.assert
+    window.assert = chai.assert
   </script>
 
   <script src="../bower_components/es6-promise/promise.js"></script>
@@ -20,7 +20,7 @@
   <script src="test.js"></script>
 
   <script>
-    if (self.mochaPhantomJS) {
+    if (window.mochaPhantomJS) {
       mochaPhantomJS.run();
     } else {
       var runner = mocha.run();
diff --git a/test/test.js b/test/test.js
index 5cdd6751..2de004ee 100644
--- a/test/test.js
+++ b/test/test.js
@@ -1,5 +1,5 @@
 function readBlobAsText(blob) {
-  if ('FileReader' in self) {
+  if ('FileReader' in window) {
     return new Promise(function(resolve, reject) {
       var reader = new FileReader()
       reader.onload = function() {
@@ -10,7 +10,7 @@ function readBlobAsText(blob) {
       }
       reader.readAsText(blob)
     })
-  } else if ('FileReaderSync' in self) {
+  } else if ('FileReaderSync' in window) {
     return new FileReaderSync().readAsText(blob)
   } else {
     throw new ReferenceError('FileReader is not defined')
@@ -18,7 +18,7 @@ function readBlobAsText(blob) {
 }
 
 function readBlobAsBytes(blob) {
-  if ('FileReader' in self) {
+  if ('FileReader' in window) {
     return new Promise(function(resolve, reject) {
       var reader = new FileReader()
       reader.onload = function() {
@@ -30,7 +30,7 @@ function readBlobAsBytes(blob) {
       }
       reader.readAsArrayBuffer(blob)
     })
-  } else if ('FileReaderSync' in self) {
+  } else if ('FileReaderSync' in window) {
     return new FileReaderSync().readAsArrayBuffer(blob)
   } else {
     throw new ReferenceError('FileReader is not defined')
@@ -205,7 +205,7 @@ suite('Request', function() {
     })
   })
 
-  ;(ArrayBuffer in self ? test : test.skip)('sends ArrayBuffer body', function() {
+  ;(ArrayBuffer in window ? test : test.skip)('sends ArrayBuffer body', function() {
     var text = 'name=Hubot'
 
     var buf = new ArrayBuffer(text.length)
@@ -834,7 +834,7 @@ suite('Atomic HTTP redirect handling', function() {
 
 // https://fetch.spec.whatwg.org/#concept-request-credentials-mode
 suite('credentials mode', function() {
-  var omitSupported = !self.fetch.polyfill
+  var omitSupported = !window.fetch.polyfill
 
   setup(function() {
     return fetch('/cookie?name=foo&value=reset', {credentials: 'same-origin'});
diff --git a/test/worker.js b/test/worker.js
index c31b4598..39e3c9d1 100644
--- a/test/worker.js
+++ b/test/worker.js
@@ -2,7 +2,7 @@ importScripts('../node_modules/chai/chai.js')
 importScripts('../node_modules/mocha/mocha.js')
 
 mocha.setup('tdd')
-self.assert = chai.assert
+window.assert = chai.assert
 
 importScripts('../bower_components/es6-promise/promise.js')
 importScripts('../fetch.js')
@@ -15,15 +15,15 @@ function title(test) {
 
 function reporter(runner) {
   runner.on('pending', function(test){
-    self.postMessage({name: 'pending', title: title(test)});
+    window.postMessage({name: 'pending', title: title(test)});
   });
 
   runner.on('pass', function(test){
-    self.postMessage({name: 'pass', title: title(test)});
+    window.postMessage({name: 'pass', title: title(test)});
   });
 
   runner.on('fail', function(test, err){
-    self.postMessage({
+    window.postMessage({
       name: 'fail',
       title: title(test),
       message: err.message,
@@ -32,7 +32,7 @@ function reporter(runner) {
   });
 
   runner.on('end', function(){
-    self.postMessage({name: 'end'});
+    window.postMessage({name: 'end'});
   });
 }