--- a/dom/base/nsDOMWindowUtils.cpp
+++ b/dom/base/nsDOMWindowUtils.cpp
@@ -3020,17 +3020,17 @@ NS_IMETHODIMP
 nsDOMWindowUtils::GetFileId(JS::Handle<JS::Value> aFile, JSContext* aCx,
                             int64_t* aResult)
 {
   MOZ_RELEASE_ASSERT(nsContentUtils::IsCallerChrome());
 
   if (!aFile.isPrimitive()) {
     JSObject* obj = aFile.toObjectOrNull();
 
-    FileHandle* fileHandle;
+    FileHandle* fileHandle = nullptr;
     if (NS_SUCCEEDED(UNWRAP_OBJECT(FileHandle, obj, fileHandle))) {
       *aResult = fileHandle->GetFileId();
       return NS_OK;
     }
 
     nsISupports* nativeObj =
       nsContentUtils::XPConnect()->GetNativeOfWrapper(aCx, obj);