Trivial attempt fails with "CoreUI's sidebar require Perfect Scrollbar"

I’ve tried a trivial example consisting of just the “Starter template” and pasted the body from the “Default layout” example into it. If I run it, I’m getting an error “CoreUI’s sidebar require Perfect Scrollbar”.

I’m not sure what I’m missing. The (somewhat reduced) code is:

<!doctype html>
<html lang="en">
    <head>
        <!-- Required meta tags -->
        <meta charset="utf-8">
        <meta name="viewport" content="width=device-width, initial-scale=1">
        <!-- CoreUI CSS -->
        <link rel="stylesheet" href="https://unpkg.com/@coreui/coreui/dist/css/coreui.min.css" crossorigin="anonymous">
        <title>CoreUI</title>
    </head>
    <body class="c-app">
        <div class="c-sidebar">
            <!-- Sidebar content here -->
        </div>
        <!-- Popper.js first, then CoreUI JS -->
        <script src="https://unpkg.com/@popperjs/core@2"></script>
        <script src="https://unpkg.com/@coreui/coreui/dist/js/coreui.min.js"></script>
    </body>
</html>