addEventListener' of null 为什么null

> > div class="mui-content-padded">  
        <button id="login" type="button" class="mui-btn mui-btn-green mui-btn-block">登 录</button>     
    </div>>

//报错addEventListener' of null，，为什么是null呀，，没有写错呀  
document.getElementById('login').addEventListener('tap',function () {  
  console.log("tap event trigger");  
});