diff --git a/src/reverseproxy.js b/src/reverseproxy.js
index de5d3678b..255218dd4 100644
--- a/src/reverseproxy.js
+++ b/src/reverseproxy.js
@@ -369,6 +369,9 @@ async function updateCertBlobs(fqdn, domainObject) {
 }
 
 async function needsRenewal(fqdn, domainObject) {
+    assert.strictEqual(typeof fqdn, 'string');
+    assert.strictEqual(typeof domainObject, 'object');
+
     const apiOptions = await getAcmeApiOptions(domainObject);
     const { certFilePath } = getAcmeCertificatePathSync(fqdn, domainObject);
 
@@ -380,6 +383,9 @@ async function needsRenewal(fqdn, domainObject) {
 }
 
 async function renewCert(fqdn, domainObject) {
+    assert.strictEqual(typeof fqdn, 'string');
+    assert.strictEqual(typeof domainObject, 'object');
+
     const apiOptions = await getAcmeApiOptions(domainObject);
     const acmePaths = getAcmeCertificatePathSync(fqdn, domainObject);
 
@@ -577,6 +583,10 @@ async function writeAppConfigs(app) {
 }
 
 async function setUserCertificate(app, fqdn, certificate) {
+    assert.strictEqual(typeof app, 'object');
+    assert.strictEqual(typeof fqdn, 'string');
+    assert.strictEqual(typeof certificate, 'object');
+
     const { certFilePath, keyFilePath } = getUserCertificatePathSync(fqdn);
 
     if (certificate !== null) {