diff --git a/dom/workers/ServiceWorker.cpp b/dom/workers/ServiceWorker.cpp
index 46129c6c3d2..45b915e3cbd 100644
--- a/dom/workers/ServiceWorker.cpp
+++ b/dom/workers/ServiceWorker.cpp
@@ -90,17 +90,22 @@ ServiceWorker::PostMessage(JSContext* aCx, JS::Handle<JS::Value> aMessage,
                            const Optional<Sequence<JS::Value>>& aTransferable,
                            ErrorResult& aRv)
 {
-  WorkerPrivate* workerPrivate = GetWorkerPrivate();
-  MOZ_ASSERT(workerPrivate);
-
   if (State() == ServiceWorkerState::Redundant) {
     aRv.Throw(NS_ERROR_DOM_INVALID_STATE_ERR);
     return;
   }
 
   nsCOMPtr<nsPIDOMWindow> window = do_QueryInterface(GetParentObject());
-  nsCOMPtr<nsIDocument> doc = window->GetExtantDoc();
-  nsAutoPtr<ServiceWorkerClientInfo> clientInfo(new ServiceWorkerClientInfo(doc));
+  if (!window || !window->GetExtantDoc()) {
+    NS_WARNING("Trying to call post message from an invalid dom object.");
+    aRv.Throw(NS_ERROR_DOM_INVALID_STATE_ERR);
+    return;
+  }
+
+  WorkerPrivate* workerPrivate = GetWorkerPrivate();
+  MOZ_ASSERT(workerPrivate);
+
+  nsAutoPtr<ServiceWorkerClientInfo> clientInfo(new ServiceWorkerClientInfo(window->GetExtantDoc()));
 
   workerPrivate->PostMessageToServiceWorker(aCx, aMessage, aTransferable,
                                             clientInfo, aRv);
diff --git a/testing/web-platform/mozilla/meta/service-workers/service-worker/fetch-event-async-respond-with.https.html.ini b/testing/web-platform/mozilla/meta/service-workers/service-worker/fetch-event-async-respond-with.https.html.ini
index f44bb2122de..dcee8d7d596 100644
--- a/testing/web-platform/mozilla/meta/service-workers/service-worker/fetch-event-async-respond-with.https.html.ini
+++ b/testing/web-platform/mozilla/meta/service-workers/service-worker/fetch-event-async-respond-with.https.html.ini
@@ -1,3 +1,5 @@
 [fetch-event-async-respond-with.https.html]
   type: testharness
-  expected: CRASH
+  expected: OK
+  [Calling respondWith asynchronously throws an exception]
+    expected: FAIL
diff --git a/testing/web-platform/mozilla/meta/service-workers/service-worker/fetch-event-respond-with-stops-propagation.https.html.ini b/testing/web-platform/mozilla/meta/service-workers/service-worker/fetch-event-respond-with-stops-propagation.https.html.ini
index 9bd19343af1..5c8e908b24b 100644
--- a/testing/web-platform/mozilla/meta/service-workers/service-worker/fetch-event-respond-with-stops-propagation.https.html.ini
+++ b/testing/web-platform/mozilla/meta/service-workers/service-worker/fetch-event-respond-with-stops-propagation.https.html.ini
@@ -1,3 +1,6 @@
 [fetch-event-respond-with-stops-propagation.https.html]
   type: testharness
-  expected: CRASH
+  expected: OK
+  [respondWith() invokes stopImmediatePropagation()]
+    expected: FAIL
+