diff --git a/lib/https/ca.js b/lib/https/ca.js
index 7afa7fb17..99faf3798 100644
--- a/lib/https/ca.js
+++ b/lib/https/ca.js
@@ -92,7 +92,7 @@ function createCertificate(hostname) {
   var serialNumber = crypto.createHash('sha1')
     .update(hostname + RANDOM_SERIAL, 'binary').digest('hex');
   if (serialNumber[0] === '0' && serialNumber[1] === '0') {
-    serialNumber = 'a' + serialNumber.substring(1);
+    serialNumber = '1' + serialNumber.substring(1);
   }
   cert = createCert(pki.setRsaPublicKey(ROOT_KEY.n, ROOT_KEY.e), serialNumber);