diff --git a/lib/get-acme-urls.js b/lib/get-acme-urls.js
index becc9bc..daf4134 100644
--- a/lib/get-acme-urls.js
+++ b/lib/get-acme-urls.js
@@ -10,6 +10,9 @@ module.exports.create = function (deps) {
   var knownUrls = deps.LeCore.knownEndpoints;
 
   function getAcmeUrls(acmeDiscoveryUrl, cb) {
+    if ('string' !== typeof acmeDiscoveryUrl) {
+      cb(new Error("getAcmeUrls: acmeDiscoveryUrl must be a string"));
+    }
 
     // TODO check response header on request for cache time
     return request({