diff --git a/lib/stringify.js b/lib/stringify.js
index b74c014c..87498203 100644
--- a/lib/stringify.js
+++ b/lib/stringify.js
@@ -147,7 +147,7 @@ var normalizeStringifyOptions = function normalizeStringifyOptions(opts) {
         return defaults;
     }
 
-    if (opts.encoder !== null && opts.encoder !== undefined && typeof opts.encoder !== 'function') {
+    if (opts.encoder !== null && typeof opts.encoder !== 'undefined' && typeof opts.encoder !== 'function') {
         throw new TypeError('Encoder has to be a function.');
     }