add() function with no arguments

Hello, I am a little confused when using the `add` function in classList.js. If I pass no argument to `add`, just like:
`element.classList.add()`, the element class will add `"undefined"` to the end.

This is because the source code using `token = tokens[i] + "";` to generate className `"undefined"` , but in HTML5 classList property, `element.classList.add()` will not affect the class name.

Of course this is an easy bug to fix, I am wondering if it is some kind of features so you leave it the way? Or it is just a bug :)

Sorry for my poor English and looking forward to your reply.