title: Rails 5, "nil is not a valid asset source"
body: <p>I have just upgraded to Rails 5 and I have a weird issue while trying to show an image.</p>
  
  <p>I have the exact code I had for Rails 4:</p>
  
  <pre><code>&lt;%= image_tag article.image_url(:thumb) %&gt;
  </code></pre>
  
  <p>But after upgrading I get:</p>
  
  <pre><code>nil is not a valid asset source
  </code></pre>
  
  <p>Before upgrading to Rails 5, <strong>I didn't have any similar issue</strong>.</p>
  
  <p>What could be at fault here?  Can it be something else and not a Rails upgrade issue?</p>