diff --git a/lib/eventsource-polyfill.js b/lib/eventsource-polyfill.js
index dcdcb66..6ed4396 100644
--- a/lib/eventsource-polyfill.js
+++ b/lib/eventsource-polyfill.js
@@ -1,2 +1,9 @@
-window.EventSourcePolyfill = require('./eventsource')
-window.EventSource = window.EventSource || window.EventSourcePolyfill
+var EventSource = require('./eventsource')
+
+if (typeof window === 'object') {
+  window.EventSourcePolyfill = EventSource
+  if (!window.EventSource) window.EventSource = EventSource
+  module.exports = window.EventSource
+} else {
+  module.exports = EventSource
+}