diff --git a/src/cert/acme2.js b/src/cert/acme2.js
index f9d1f1c23..27f3a0d1a 100644
--- a/src/cert/acme2.js
+++ b/src/cert/acme2.js
@@ -47,12 +47,12 @@ function urlBase64Encode(string) {
 }
 
 function b64(str) {
-    var buf = util.isBuffer(str) ? str : Buffer.from(str);
+    var buf = Buffer.isBuffer(str) ? str : Buffer.from(str);
     return urlBase64Encode(buf.toString('base64'));
 }
 
 function getModulus(pem) {
-    assert(util.isBuffer(pem));
+    assert(Buffer.isBuffer(pem));
 
     var stdout = safe.child_process.execSync('openssl rsa -modulus -noout', { input: pem, encoding: 'utf8' });
     if (!stdout) return null;
@@ -301,7 +301,7 @@ Acme2.prototype.waitForChallenge = function (challenge, callback) {
 Acme2.prototype.signCertificate = function (domain, finalizationUrl, csrDer, callback) {
     assert.strictEqual(typeof domain, 'string');
     assert.strictEqual(typeof finalizationUrl, 'string');
-    assert(util.isBuffer(csrDer));
+    assert(Buffer.isBuffer(csrDer));
     assert.strictEqual(typeof callback, 'function');
 
     const payload = {
diff --git a/src/settings.js b/src/settings.js
index f80af8d62..d3fbaab3c 100644
--- a/src/settings.js
+++ b/src/settings.js
@@ -307,7 +307,7 @@ function getCloudronAvatar(callback) {
 }
 
 function setCloudronAvatar(avatar, callback) {
-    assert(util.isBuffer(avatar));
+    assert(Buffer.isBuffer(avatar));
     assert.strictEqual(typeof callback, 'function');
 
     if (!safe.fs.writeFileSync(paths.CLOUDRON_AVATAR_FILE, avatar)) {