diff --git a/dom/workers/WorkerScope.cpp b/dom/workers/WorkerScope.cpp
index dc7482aa41f9..807bbd26f250 100644
--- a/dom/workers/WorkerScope.cpp
+++ b/dom/workers/WorkerScope.cpp
@@ -984,11 +984,13 @@ CreateDedicatedWorkerGlobalScope(JSContext* aCx)
     return NULL;
   }
 
-  // Init other paris-bindings.
-  if (!XMLHttpRequestBinding_workers::CreateInterfaceObjects(aCx, global,
-                                                             global) ||
-      !XMLHttpRequestUploadBinding_workers::CreateInterfaceObjects(aCx, global,
-                                                                   global)) {
+  // Init other paris-bindings.  Use GetProtoObject so the proto will
+  // be correctly cached in the proto cache.  Otherwise we'll end up
+  // double-calling CreateInterfaceObjects when we actually create an
+  // object which has these protos, which breaks things like
+  // instanceof.
+  if (!XMLHttpRequestBinding_workers::GetProtoObject(aCx, global, global) ||
+      !XMLHttpRequestUploadBinding_workers::GetProtoObject(aCx, global, global)) {
     return NULL;
   }
 
diff --git a/dom/workers/test/Makefile.in b/dom/workers/test/Makefile.in
index 157cd5b37c90..b1a3f3e559a3 100644
--- a/dom/workers/test/Makefile.in
+++ b/dom/workers/test/Makefile.in
@@ -39,6 +39,8 @@ MOCHITEST_FILES = \
   importScripts_worker_imported2.js \
   importScripts_worker_imported3.js \
   importScripts_worker_imported4.js \
+  test_instanceof.html \
+  instanceof_worker.js \
   test_json.html \
   json_worker.js \
   test_location.html \
diff --git a/dom/workers/test/instanceof_worker.js b/dom/workers/test/instanceof_worker.js
new file mode 100644
index 000000000000..a98255388f75
--- /dev/null
+++ b/dom/workers/test/instanceof_worker.js
@@ -0,0 +1,12 @@
+/**
+ * Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/
+ */
+onmessage = function(event) {
+  postMessage({event: "XMLHttpRequest",
+               status: (new XMLHttpRequest() instanceof XMLHttpRequest),
+               last: false });
+  postMessage({event: "XMLHttpRequestUpload",
+               status: ((new XMLHttpRequest()).upload instanceof XMLHttpRequestUpload),
+               last: true });
+}
diff --git a/dom/workers/test/test_instanceof.html b/dom/workers/test/test_instanceof.html
new file mode 100644
index 000000000000..f73b3b6a71bb
--- /dev/null
+++ b/dom/workers/test/test_instanceof.html
@@ -0,0 +1,40 @@
+<!--
+  Any copyright is dedicated to the Public Domain.
+  http://creativecommons.org/publicdomain/zero/1.0/
+-->
+<!DOCTYPE HTML>
+<html>
+<!--
+Tests of DOM Worker JSON messages
+-->
+<head>
+  <title>Test for DOM Worker Navigator</title>
+  <script type="text/javascript" src="/tests/SimpleTest/SimpleTest.js"></script>
+  <link rel="stylesheet" type="text/css" href="/tests/SimpleTest/test.css" />
+</head>
+<body>
+<p id="display"></p>
+<div id="content" style="display: none">
+
+</div>
+<pre id="test">
+<script src="json_worker.js" language="javascript"></script>
+<script class="testbody" language="javascript">
+
+  var worker = new Worker("instanceof_worker.js");
+
+  worker.onmessage = function(event) {
+    ok(event.data.status, event.data.event);
+
+    if (event.data.last)
+      SimpleTest.finish();
+  };
+
+  worker.postMessage(42);
+
+  SimpleTest.waitForExplicitFinish();
+
+</script>
+</pre>
+</body>
+</html>
