diff --git a/lib/challenge-handlers.js b/lib/challenge-handlers.js
index eb08000..0bb463a 100644
--- a/lib/challenge-handlers.js
+++ b/lib/challenge-handlers.js
@@ -26,7 +26,7 @@ module.exports = {
       if (args.debug) {
         console.debug('[LEX] mkdirp', webrootPath);
       }
-      mkdirp(webrootPath, function () {
+      mkdirp(webrootPath, function (err) {
         if (err) { cb(err); return; }
 
         fs.writeFile(keyfile, value, 'utf8', cb);