diff --git a/lib/tls.js b/lib/tls.js
index 0d85a948dc..8bd9991d87 100644
--- a/lib/tls.js
+++ b/lib/tls.js
@@ -2,7 +2,6 @@
 
 const net = require('net');
 const url = require('url');
-const util = require('util');
 const binding = process.binding('crypto');
 const Buffer = require('buffer').Buffer;
 const constants = require('constants');
@@ -126,9 +125,7 @@ exports.checkServerIdentity = function checkServerIdentity(host, cert) {
       return ip === host;
     });
     if (!valid) {
-      reason = util.format('IP: %s is not in the cert\'s list: %s',
-                           host,
-                           ips.join(', '));
+      reason = `IP: ${host} is not in the cert's list: ${ips.join(', ')}`;
     }
   } else if (cert.subject) {
     // Transform hostname to canonical form
@@ -174,13 +171,11 @@ exports.checkServerIdentity = function checkServerIdentity(host, cert) {
 
     if (!valid) {
       if (cert.subjectaltname) {
-        reason = util.format('Host: %s is not in the cert\'s altnames: %s',
-                             host,
-                             cert.subjectaltname);
+        reason =
+            `Host: ${host} is not in the cert's altnames: ` +
+            `${cert.subjectaltname}`;
       } else {
-        reason = util.format('Host: %s is not cert\'s CN: %s',
-                             host,
-                             cert.subject.CN);
+        reason = `Host: ${host} is not cert's CN: ${cert.subject.CN}`;
       }
     }
   } else {
@@ -189,8 +184,7 @@ exports.checkServerIdentity = function checkServerIdentity(host, cert) {
 
   if (!valid) {
     var err = new Error(
-        util.format('Hostname/IP doesn\'t match certificate\'s altnames: %j',
-                    reason));
+        `Hostname/IP doesn't match certificate's altnames: "${reason}"`);
     err.reason = reason;
     err.host = host;
     err.cert = cert;