diff --git a/content/events/src/nsDOMEvent.cpp b/content/events/src/nsDOMEvent.cpp
index 003195108cea..7558680c661a 100644
--- a/content/events/src/nsDOMEvent.cpp
+++ b/content/events/src/nsDOMEvent.cpp
@@ -578,7 +578,7 @@ nsDOMEvent::DuplicatePrivateData()
     case NS_SCRIPT_ERROR_EVENT:
     {
       InternalScriptErrorEvent* oldScriptErrorEvent =
-        static_cast<InternalScriptErrorEvent*>(mEvent);
+        mEvent->AsScriptErrorEvent();
       InternalScriptErrorEvent* scriptErrorEvent =
         new InternalScriptErrorEvent(false, msg);
       scriptErrorEvent->AssignScriptErrorEventData(*oldScriptErrorEvent, true);
diff --git a/dom/src/events/nsJSEventListener.cpp b/dom/src/events/nsJSEventListener.cpp
index 60658b256e1e..2cafd966684f 100644
--- a/dom/src/events/nsJSEventListener.cpp
+++ b/dom/src/events/nsJSEventListener.cpp
@@ -176,11 +176,9 @@ nsJSEventListener::HandleEvent(nsIDOMEvent* aEvent)
     Optional<uint32_t> columnNumber;
 
     NS_ENSURE_TRUE(aEvent, NS_ERROR_UNEXPECTED);
-    WidgetEvent* event = aEvent->GetInternalNSEvent();
-    if (event->message == NS_LOAD_ERROR &&
-        event->eventStructType == NS_SCRIPT_ERROR_EVENT) {
-      InternalScriptErrorEvent *scriptEvent =
-        static_cast<InternalScriptErrorEvent*>(event);
+    InternalScriptErrorEvent* scriptEvent =
+      aEvent->GetInternalNSEvent()->AsScriptErrorEvent();
+    if (scriptEvent && scriptEvent->message == NS_LOAD_ERROR) {
       errorMsg = scriptEvent->errorMsg;
       msgOrEvent.SetAsString() = static_cast<nsAString*>(&errorMsg);