diff --git a/src/core/worker.js b/src/core/worker.js
index 1abe28de0..eeea840c2 100644
--- a/src/core/worker.js
+++ b/src/core/worker.js
@@ -438,7 +438,7 @@ function setPDFNetworkStreamClass(cls) {
 }
 
 var WorkerMessageHandler = {
-  setup: function wphSetup(handler, port) {
+  setup(handler, port) {
     var testMessageProcessed = false;
     handler.on('test', function wphSetupTest(data) {
       if (testMessageProcessed) {
@@ -481,7 +481,7 @@ var WorkerMessageHandler = {
       return WorkerMessageHandler.createDocumentHandler(data, port);
     });
   },
-  createDocumentHandler: function wphCreateDocumentHandler(docParams, port) {
+  createDocumentHandler(docParams, port) {
     // This context is actually holds references on pdfManager and handler,
     // until the latter is destroyed.
     var pdfManager;
@@ -956,18 +956,23 @@ var WorkerMessageHandler = {
       docParams = null; // we don't need docParams anymore -- saving memory.
     });
     return workerHandlerName;
-  }
+  },
+  initializeFromPort(port) {
+    var handler = new MessageHandler('worker', 'main', port);
+    WorkerMessageHandler.setup(handler, port);
+    handler.send('ready', null);
+  },
 };
 
-function initializeWorker() {
-  var handler = new MessageHandler('worker', 'main', self);
-  WorkerMessageHandler.setup(handler, self);
-  handler.send('ready', null);
+function isMessagePort(maybePort) {
+  return typeof maybePort.postMessage === 'function' &&
+         ('onmessage' in maybePort);
 }
 
 // Worker thread (and not node.js)?
-if (typeof window === 'undefined' && !isNodeJS()) {
-  initializeWorker();
+if (typeof window === 'undefined' && !isNodeJS() &&
+    typeof self !== 'undefined' && isMessagePort(self)) {
+  WorkerMessageHandler.initializeFromPort(self);
 }
 
 exports.setPDFNetworkStreamClass = setPDFNetworkStreamClass;