bootstrap 4.1 sticky footer not working code example

Example: sticky footer not working bootstrap

/* 2020 - bootstrap 4.5+ 
Note: - The flex-fill utility was included in Bootstrap 4.1 at later release. 
So after that release the extra CSS for flex-fill won't be needed. 
Additionally min-vh-100 is included in newer Bootstrap 4 releases */
<div class="d-flex flex-column min-vh-100">
    <nav>
    </nav>
    <main class="flex-fill">
    </main>
    <footer>
    </footer>
</div>