diff --git a/dnsconfig.js b/dnsconfig.js
index faf5db4900..04c4b866a9 100644
--- a/dnsconfig.js
+++ b/dnsconfig.js
@@ -172,17 +172,17 @@ for (var subdomain in domains) {
     }
 }
 
-var existingSubdomains = new Set();
+var existingSubdomains = {};
 var eligibleDomains = [];
 
 for (var i = 0; i < domains.length; i++) {
     var subdomainName = domains[i].name;
     var data = domains[i].data;
 
-    existingSubdomains.add(subdomainName);
+    existingSubdomains[subdomainName] = true;
 
     if (
-        !subdomainName.startsWith("www.") &&
+        subdomainName.substring(0, 4) !== "www." &&
         (data.records.A || data.records.AAAA || data.records.CNAME || data.records.URL)
     ) {
         eligibleDomains.push(subdomainName);
@@ -193,7 +193,7 @@ for (var i = 0; i < eligibleDomains.length; i++) {
     var sub = eligibleDomains[i];
     var wwwSub = "www." + sub;
 
-    if (!existingSubdomains.has(wwwSub)) {
+    if (!existingSubdomains[wwwSub]) {
         records.push(A(wwwSub, IP("192.0.2.1"), CF_PROXY_ON));
     }
 }