$app->sendFile works when in debug mode, fails otherwise

Question

I have this route in my Silex router:

$app->get( "/route", function () use ( $app ) {
    $path = __DIR__ . "/../path/to/files/dir";
    $files=glob("$path/*.*");
    sort($files);
    $files = array_reverse($files);
    $newestFile = $files[0];
    $newestBasename = basename($newestFile);

    return $app
        ->sendFile( $newestFile )
        ->setContentDisposition( ResponseHeaderBag::DISPOSITION_ATTACHMENT, $newestBasename );
});

It's fairly straightforward, copied basically verbatim from Silex' docs. It works fine when I'm in debug mode; but, as soon as I turn off debug mode, it gives me a "Whoops, something went wrong" error message when I hit the route.

I've tried:

• Removing the setContentDisposition line.
• Streaming the file instead of using sendFile.
• Reading other people's questions about sendFile (1, 2, 3, 4).

All to no avail.

What am I doing wrong?

[ Note regarding symfony tag: I'm not using Symfony per-se, but included the tag since I am using the Symfony component ResponseHeaderBag as part of this route. ]