diff --git a/src/util.js b/src/util.js
index dbc8de3f..3a1f5f7e 100644
--- a/src/util.js
+++ b/src/util.js
@@ -88,7 +88,7 @@ export default {
           }
           return;
         }
-        if (typeof MessagePort !== 'undefined' && MessagePort.prototype.isPrototypeOf(value)) {
+        if (Object.prototype.toString.call(value) === '[object MessagePort]') {
           throw new Error("Can't transfer the same stream twice.");
         }
         util.collectTransferables(value, collection);
@@ -104,7 +104,7 @@ export default {
   restoreStreams: function(obj) {
     if (Object.prototype.isPrototypeOf(obj) && !Uint8Array.prototype.isPrototypeOf(obj)) {
       Object.entries(obj).forEach(([key, value]) => { // recursively search all children
-        if (MessagePort.prototype.isPrototypeOf(value)) {
+        if (Object.prototype.toString.call(value) === '[object MessagePort]') {
           obj[key] = new ReadableStream({
             pull(controller) {
               return new Promise(resolve => {