--- a/services/sync/modules/engines/prefs.js
+++ b/services/sync/modules/engines/prefs.js
@@ -17,17 +17,18 @@ Cu.import("resource://services-sync/engi
 Cu.import("resource://services-sync/record.js");
 Cu.import("resource://services-sync/util.js");
 Cu.import("resource://services-sync/constants.js");
 Cu.import("resource://services-common/utils.js");
 
 XPCOMUtils.defineLazyModuleGetter(this, "LightweightThemeManager",
                           "resource://gre/modules/LightweightThemeManager.jsm");
 
-const PREFS_GUID = CommonUtils.encodeBase64URL(Services.appinfo.ID);
+XPCOMUtils.defineLazyGetter(this, "PREFS_GUID",
+                            () => CommonUtils.encodeBase64URL(Services.appinfo.ID));
 
 this.PrefRec = function PrefRec(collection, id) {
   CryptoWrapper.call(this, collection, id);
 }
 PrefRec.prototype = {
   __proto__: CryptoWrapper.prototype,
   _logName: "Sync.Record.Pref",
 };