--- a/toolkit/components/antitracking/test/browser/browser_blockingServiceWorkers.js
+++ b/toolkit/components/antitracking/test/browser/browser_blockingServiceWorkers.js
@@ -1,15 +1,16 @@
 requestLongerTimeout(2);
 
 AntiTracking.runTest("ServiceWorkers",
   async _ => {
     await navigator.serviceWorker.register("empty.js").then(
       _ => { ok(false, "ServiceWorker cannot be used!"); },
-      _ => { ok(true, "ServiceWorker cannot be used!"); });
+      _ => { ok(true, "ServiceWorker cannot be used!"); }).
+      catch(e => ok(false, "Promise rejected: " + e));
   },
   null,
   async _ => {
     await new Promise(resolve => {
       Services.clearData.deleteData(Ci.nsIClearDataService.CLEAR_ALL, value => resolve());
     });
   },
   [["dom.serviceWorkers.exemptFromPerDomainMax", true],