diff --git a/lib/server.js b/lib/server.js
index 2ef676a..89da85e 100644
--- a/lib/server.js
+++ b/lib/server.js
@@ -259,7 +259,7 @@ function Server(options) {
     if (options.certificate || options.key) {
       if (!(options.certificate && options.key) ||
           (typeof (options.certificate) !== 'string' &&
-          !Buffer.isBuffer(options.key)) ||
+          !Buffer.isBuffer(options.certificate)) ||
           (typeof (options.key) !== 'string' &&
           !Buffer.isBuffer(options.key))) {
         throw new TypeError('options.certificate and options.key ' +