title: {PreviousPost} and {NextPost} not working on Tumblr
body: <p>It seems like {PreviousPost} and {NextPost} is not working on Tumblr for creating custom themes.  Here's my code. Can someone point to my mistake...</p>
  
  <pre><code>&lt;div&gt;
      {block:PreviousPost}&lt;a href="{PreviousPost}"&gt;&lt;span&gt;Previous Post&lt;/span&gt;&lt;/a&gt;{/block:PreviousPost}
      {block:NextPost}&lt;a href="{NextPost}"&gt;&lt;span&gt;Next Post&lt;/span&gt;&lt;/a&gt;{/block:NextPost}
  &lt;/div&gt; 
  </code></pre>