title: A non-empty PSR-4 prefix must end with a namespace separator
body: <p>I'm trying to setup PSR-4 with Composer but I'm just getting <code>A non-empty PSR-4 prefix must end with a namespace separator.</code></p>
  
  <p>My <code>autoload</code> in my <code>composer.json</code> looks like this:</p>
  
  <pre><code>"autoload": {
      "psr-4": {
          "Acme\\models" : "app/models"
      }
  },
  </code></pre>
  
  <p><code>app/models</code> is empty.</p>
  
  <p>What am I doing wrong? How can I fix this?</p>