diff --git a/index.js b/index.js
index b08cca4..8065299 100644
--- a/index.js
+++ b/index.js
@@ -39,6 +39,7 @@ falsey.keywords = [
   'nix',
   'nyet',
   'uh-uh',
+  'undefined',
   'veto',
   'zero'
 ];