title: Incompatible units: 'rem' and 'px' with bootstrap 4 alpha 6
body: <p>I am trying to implement a bootstrap 4 template (I use bootstrap 4 alpha 6) and see such an error:</p>
  
  <pre><code>Incompatible units: 'rem' and 'px'.
  </code></pre>
  
  <p>in line </p>
  
  <pre><code>$input-height: (($font-size-base * $input-btn-line-height) + ($input-btn-padding-y * 2)) !default;
  </code></pre>
  
  <p>Did somebody experience similar issue?
  Thanks!</p>