diff --git a/dom/interfaces/base/nsIContentPrefService2.idl b/dom/interfaces/base/nsIContentPrefService2.idl
index da8fe7146432..edbcbc0bd2bb 100644
--- a/dom/interfaces/base/nsIContentPrefService2.idl
+++ b/dom/interfaces/base/nsIContentPrefService2.idl
@@ -67,7 +67,7 @@ interface nsIContentPref;
  * See nsIContentPrefCallback2 below for more information about callbacks.
  */
 
-[scriptable, uuid(133608c7-f812-41ca-bc1c-62a4eb95e52a)]
+[scriptable, uuid(86279644-6b86-4875-a228-2d2ff2f3e33b)]
 interface nsIContentPrefService2 : nsISupports
 {
   /**
@@ -153,7 +153,7 @@ interface nsIContentPrefService2 : nsISupports
   void getCachedBySubdomainAndName(in AString domain,
                                    in AString name,
                                    in nsILoadContext context,
-                                   out unsigned long len,
+                                   [optional] out unsigned long len,
                                    [retval,array,size_is(len)] out nsIContentPref prefs);
 
   /**