How to conditionally include a sub-app with Express?

I'm using a library called Grant in my application. Previously I have included it following the examples in the readme linked above, however I now have the need to conditionally include it and I can't seem to make it work.

How it worked before

const grant = new Grant(grantConfig);
app.use(grant);

I've done conditional middleware in Express before, so I thought including Grant wouldn't be a problem.

How I Tried (doesn't work)

const grant = new Grant(grantConfig);

app.use((req, res, next) => {
    if (someBooleanVariable) {
        next();
    } else {
        grant(req, res, next);
    }
});

So this doesn't work. I think it may have something to do with Grant being an instance of Express versus just a regular middleware function, but I'm not sure. You can see how Grant is implemented here. The Express docs say that it should be treated the same, but I might also be misunderstanding them.

via Jack Slingerland