when i add the classes "navbar-dark bg-dark" into my navbar, navbar's item isn't showing

Question

In the First line when add last two classes(nav-dark,bg-dark). navbar's items are not showing

<nav class="navbar navbar-expand-lg nav-dark bg-dark">
<ul class="navbar-nav">
    <li class="nav-item">
        Download
    </li>
    <li class="nav-item">
        Pricing
    </li>
    <li class="nav-item">
        Menu
    </li>
</ul>