title: Cannot read property 'className' of null
body: <p>This isn't working. I have an element in my html with the id "color". What is the deal? I just want it to add to the class of that element.</p>
  
  <pre><code>var el = document.getElementById("color");
  var number = Math.floor((Math.random() * 5) + 1);
  
  switch (number) {
      case 1:
          el.className += " blue";
          break;
  
      case 2:
          el.className += " yellow";
          break;
  
      case 3:
          el.className += " red";
          break;
  
      case 4:
          el.className += " green";
          break;
  
      case 5:
          el.className += " purple";
          break;
  }
  </code></pre>