diff --git a/src/zen/common/ZenUIManager.mjs b/src/zen/common/ZenUIManager.mjs
index 2b2834b94..002c3a95e 100644
--- a/src/zen/common/ZenUIManager.mjs
+++ b/src/zen/common/ZenUIManager.mjs
@@ -531,8 +531,8 @@ var gZenVerticalTabsManager = {
 
     ChromeUtils.defineLazyGetter(this, 'hidesTabsToolbar', () => {
       return (
-        document.documentElement.getAttribute('chromehidden').includes('toolbar') ||
-        document.documentElement.getAttribute('chromehidden').includes('menubar')
+        document.documentElement.getAttribute('chromehidden')?.includes('toolbar') ||
+        document.documentElement.getAttribute('chromehidden')?.includes('menubar')
       );
     });