diff --git a/index.js b/index.js
index 5707ada..2f12988 100644
--- a/index.js
+++ b/index.js
@@ -24,7 +24,7 @@ module.exports = function(options) {
   'use strict';
 
   var opts = options ? clone(options) : {};
-  opts.ext = opts.ext || ".html";
+  opts.ext = opts.ext != null ? opts.ext : ".html";
 
   if (opts.defaults) {
     swig.setDefaults(opts.defaults);