--- a/toolkit/components/search/tests/xpcshell/head_search.js
+++ b/toolkit/components/search/tests/xpcshell/head_search.js
@@ -241,17 +241,18 @@ function getDefaultEngineName(isUS) {
     loadUsingSystemPrincipal: true
   });
   let searchSettings = parseJsonFromStream(chan.open2());
   let defaultEngineName = searchSettings.default.searchDefault;
 
   if (isUS === undefined)
     isUS = Services.locale.getRequestedLocale() == "en-US" && isUSTimezone();
 
-  if (isUS && ("searchDefault" in searchSettings.US)) {
+  if (isUS && ("US" in searchSettings &&
+               "searchDefault" in searchSettings.US)) {
     defaultEngineName = searchSettings.US.searchDefault;
   }
   return defaultEngineName;
 }
 
 /**
  * Waits for the cache file to be saved.
  * @return {Promise} Resolved when the cache file is saved.