--- a/toolkit/modules/LightweightThemeConsumer.jsm
+++ b/toolkit/modules/LightweightThemeConsumer.jsm
@@ -66,17 +66,17 @@ LightweightThemeConsumer.prototype = {
     if (aTopic != "lightweight-theme-styling-update")
       return;
 
     const { outerWindowID } = this._win
       .QueryInterface(Ci.nsIInterfaceRequestor)
       .getInterface(Ci.nsIDOMWindowUtils);
 
     const parsedData = JSON.parse(aData);
-    if (parsedData.window && parsedData.window !== outerWindowID) {
+    if (parsedData && parsedData.window && parsedData.window !== outerWindowID) {
       return;
     }
 
     this._update(parsedData);
   },
 
   handleEvent(aEvent) {
     let {width, height} = this._win.screen;