diff --git a/src/index.js b/src/index.js
index a1b21b0..761cd9a 100644
--- a/src/index.js
+++ b/src/index.js
@@ -38,19 +38,12 @@ export function polyfill(options) {
     const fetch = require('fetch-cookie')(require('isomorphic-fetch'), cookieJar);
     global.fetch = fetch;
     global.cookieJar = cookieJar;
-    global.fetchMock = undefined;
-    const { Response, Headers, Request } = require('whatwg-fetch');
-    global.Response = Response;
-    global.Headers = Headers;
-    global.Request = Request;
+    require('whatwg-fetch');
   }
 
-  if (opts.fetch) {
-    const { Response, Headers, Request } = require('whatwg-fetch');
+  if (opts.fetch && !opts.isomorphicFetch) {
+    require('whatwg-fetch');
     global.fetchMock = require('fetch-mock');
-    global.Response = Response;
-    global.Headers = Headers;
-    global.Request = Request;
   }
 
   if (opts.localStorage && !global.localStorage) {
diff --git a/src/tests/index.test.js b/src/tests/index.test.js
index 69fc322..b945e97 100644
--- a/src/tests/index.test.js
+++ b/src/tests/index.test.js
@@ -24,6 +24,7 @@ describe('polyfill', () => {
     expect(global.Response).not.toBeDefined();
     polyfill({
       media: false,
+      isomorphicFetch: false,
       fetch: true,
       localStorage: false,
     });
@@ -38,7 +39,10 @@ describe('polyfill', () => {
 
   it('should polyfill fetch', (done) => {
     polyfill({
+      media: false,
       isomorphicFetch: true,
+      fetch: false,
+      localStorage: false,
     });
     expect(global.Headers).toBeDefined();
     expect(global.Request).toBeDefined();
@@ -51,6 +55,9 @@ describe('polyfill', () => {
 
   it('should not polyfill localStorage', () => {
     polyfill({
+      media: false,
+      isomorphicFetch: false,
+      fetch: false,
       localStorage: false,
     });
     expect(global.localStorage).toBeDefined();