title: what is causing Less::Error 'Unrecognised input'?
body: <p>I'm following <a href="https://stackoverflow.com/a/50086486/2009652">this answer</a> about using yarn to set up semantic-ui with rails but get this vague error that says only <code>Unrecognised input</code> whenever I access localhost. The stack trace is short and the only clue it provides is to the stylesheet and javascript link tags in my application.html.erb head:</p>
  
  <pre><code>&lt;%= stylesheet_link_tag 'application', media: 'all', 'data-turbolinks-track' =&gt; true %&gt;
  &lt;%= javascript_include_tag 'application', 'data-turbolinks-track' =&gt; true %&gt;
  </code></pre>
  
  <p>What does <code>Unrecognised input</code> mean, and what causes it?</p>