diff --git a/chrome/content/zotero/zoteroPane.js b/chrome/content/zotero/zoteroPane.js
index edb8ef0aeb..1c148a076e 100644
--- a/chrome/content/zotero/zoteroPane.js
+++ b/chrome/content/zotero/zoteroPane.js
@@ -2439,7 +2439,8 @@ var ZoteroPane = new function()
 				|| Zotero.Prefs.get('sync.autoSync')
 				|| Zotero.Libraries.getAll()
 					.every(library => !library.syncable
-						|| library.lastSync?.getTime() > Date.now() - 1000 * sevenDays)) {
+						|| (library.lastSync
+							&& library.lastSync.getTime() > Date.now() - 1000 * sevenDays))) {
 			return;
 		}