title: Error in SDTT: "SiteNavigationElement is not a known valid target type for the additionalType property."
body: <p>I tried a simple example, but the <code>SiteNavigationElement</code> is not working when I test it using the <a href="https://search.google.com/structured-data/testing-tool/u/0/" rel="nofollow noreferrer">Google Structured Data Testing Tool</a>. It gives the error:</p>
  
  <blockquote>
    <p><code>SiteNavigationElement</code> is not a known valid target type for the <code>additionalType</code> property.</p>
  </blockquote>
  
  <p>The Microdata:</p>
  
  <pre class="lang-html prettyprint-override"><code>&lt;div itemscope itemtype="http://schema.org/WebPageElement"&gt;
    &lt;link itemprop="additionalType" href="http://schema.org/ItemList" /&gt;
    &lt;meta itemprop="name" content="navigation_menu" /&gt;
    &lt;ul&gt;
  
      &lt;li itemprop="additionalType" itemscope itemtype="http://www.schema.org/SiteNavigationElement"&gt;
        &lt;span itemprop="itemListElement"&gt;
          &lt;a href="http://www.example.com/link_1" itemprop="url"&gt;
            &lt;span itemprop="name"&gt;Link 1&lt;/span&gt;
          &lt;/a&gt;
        &lt;/span&gt;
      &lt;/li&gt;
  
      &lt;li itemprop="additionalType" itemscope itemtype="http://www.schema.org/SiteNavigationElement"&gt;
        &lt;span itemprop="itemListElement"&gt;
          &lt;a href="http://www.example.com/link_2" itemprop="url"&gt;
            &lt;span itemprop="name"&gt;Link 2&lt;/span&gt;
          &lt;/a&gt;
        &lt;/span&gt;
      &lt;/li&gt;
  
    &lt;/ul&gt;
  &lt;/div&gt;
  </code></pre>