--- a/js/xpconnect/loader/mozJSSubScriptLoader.cpp
+++ b/js/xpconnect/loader/mozJSSubScriptLoader.cpp
@@ -405,22 +405,22 @@ mozJSSubScriptLoader::ReadScriptAsync(ns
 {
     RootedObject target_obj(RootingCx(), targetObjArg);
 
     nsCOMPtr<nsIGlobalObject> globalObject = xpc::NativeGlobal(target_obj);
     ErrorResult result;
 
     AutoJSAPI jsapi;
     if (NS_WARN_IF(!jsapi.Init(globalObject))) {
-      return NS_ERROR_UNEXPECTED;
+        return NS_ERROR_UNEXPECTED;
     }
 
     RefPtr<Promise> promise = Promise::Create(globalObject, result);
     if (result.Failed()) {
-      promise = nullptr;
+        return result.StealNSResult();
     }
 
     DebugOnly<bool> asJS = ToJSValue(jsapi.cx(), promise, retval);
     MOZ_ASSERT(asJS, "Should not fail to convert the promise to a JS value");
 
     // We create a channel and call SetContentType, to avoid expensive MIME type
     // lookups (bug 632490).
     nsCOMPtr<nsIChannel> channel;