自定义函数传递了我的参数，但没有调用querySelector？

Question

运行querySelector时我什么也没得到。当我尝试登录el时，它可以工作，但是查询选择器不起作用！我最后移动了js文件，但仍无法正常工作

El正在传递，但是我的查询选择器不起作用！

(function(global) {

  let SimpleShow = function(el) {
    return new SimpleShow.init(el);
  }

  SimpleShow.init = function(el) {
    this.slider = document.querySelector(el);

    //This code isn't working. Nothing is being selected
    console.log(this.slider);
    //However this is working. El is being passed around
    console.log(el);
  }

  global.SimpleShow = SimpleShow;
}(window));


let slideshow = SimpleShow("#gallery");

ul {
  list-style-type: none;
}

.box1 {
  height: 400px;
  width: 700px;
  background: red;
}

.box2 {
  height: 400px;
  width: 700px;
  background: black;
}

.box3 {
  height: 400px;
  width: 700px;
  background: green;
}

.box4 {
  height: 400px;
  width: 700px;
  background: wheat;
}

<div id="gallery">
  <ul>
    <li class="box1"></li>
    <li class="box2"></li>
    <li class="box3"></li>
    <li class="box4"></li>
  </ul>
</div>