FontAwesome envelope icon is not displayed

[Ajax30]

Using the envelope icon from FontAwesome does not work. It just shows an empty disc (circle).

<li class="list-inline-item">
    <a href="mailto:{{ $owner_email }}" target="_blank">
      <span class="fa-stack fa-lg">
        <i class="fas fa-circle fa-stack-2x"></i>
        <i class="fab fa-envelope fa-stack-1x fa-inverse"></i>
      </span>
    </a>
  </li>

[N6REJ]

thats because envelope is fas not fab. It's not a brand