throw an Error if no DOM element is given

index.js

@@ -1,4 +1,3 @@
-
 /**
  * Module dependencies.
  */
@@ -37,6 +36,7 @@ module.exports = function(el){
  */
 
 function ClassList(el) {
+  if (!el) throw new Error('A DOM element reference is required');
   this.el = el;
   this.list = el.classList;
 }