diff --git a/browser/components/readinglist/test/xpcshell/test_scheduler.js b/browser/components/readinglist/test/xpcshell/test_scheduler.js
index bb258a4720d..8b2eb033366 100644
--- a/browser/components/readinglist/test/xpcshell/test_scheduler.js
+++ b/browser/components/readinglist/test/xpcshell/test_scheduler.js
@@ -30,7 +30,7 @@ function createScheduler(options) {
   // avoid typos in the test and other footguns in the options.
   let allowedOptions = ["expectedDelay", "expectNewTimer", "syncFunction"];
   for (let key of Object.keys(options)) {
-    if (!allowedOptions.includes(key)) {
+    if (allowedOptions.indexOf(key) == -1) {
       throw new Error("Invalid option " + key);
     }
   }