Undefined error while trying to route to client folder using express

I'm trying to sendFile index.html from inside my client directory which is at the same level as my server directory, and getting the following error:

TypeError: undefined is not a function
at Object.handle (/myapp/server/routes/routes.js:31:7)

This is line 31 of my routes.js: res.sendFile(fileName, options, function (err) {

Full function

dashRouter.get('/', function(req, res) {

    // res.send('dashboard!');

    var fileName = 'index.html';

    var options = {
        root: __dirname + '../client/dashboard/',
        dotfiles: 'deny',
        headers: {
            'x-timestamp': Date.now(),
            'x-sent': true
        }
    };

    res.sendFile(fileName, options, function (err) {
        if (err) {
            console.log(err);
            res.status(err.status).end();
        }
        else {
            console.log('Sent:', fileName);
        }
    });
});

return dashRouter;

My folder structure:

Do you know why I'm getting that undefined error on the sendFile line?