Basic node error: app.use() requires a middleware function - node.js

Question

I'm new to node and am having some probably very basic trouble with an error message that I cannot fix:
proj/node_modules/express/lib/application.js:210
throw new TypeError('app.use() requires a middleware function')
I've been trying to use the csrf token module but for some reason I'm getting the above error code - literally clueless as to finding a fix here. May be very obvious to some - does anyone have any ideas?
This is the offending line of code: app.use(csrfProtection); // ERROR HERE: REASON UNKNOWN
I have installed the csurf module exactly identically to the tutorial I'm following. Any help would be much appreciated - original code is below:
const express = require('express');
const path = require('path');
const app = express();
const bodyParser = require('body-parser'); // enables use of body parser - body parser helps get data entered from a form (via req.body)
const subRoutes = require('./routes/subs');
const adminRoutes = require('./routes/admin');
const authRoutes = require('./routes/auth');
const mongoConnect = require('./util/database').mongoConnect;
let mailActivator = require('./util/nodemailer'); // activates nodemailer.js
const session = require('express-session');
const MongoDBStore = require('connect-mongodb-session')(session); // session argument from express-session
const csrf = require('csrf');
const store = new MongoDBStore({
uri: 'mongodb+srv://_____:______#empool-3klmr.mongodb.net/______',
collection: 'sessions'
});
const csrfProtection = csrf();
app.set('view engine', 'ejs'); // use EJS templating engine
app.set('views', 'views'); // views located here
app.use(bodyParser.urlencoded({
extended: false
})); // parses text as URL encoded data (which is how browsers tend to send form data from regular forms set to POST) and exposes the resulting object (containing the keys and values) on req.body)
app.use(bodyParser.json()); // for JSON post API requests
app.use(express.static(path.join(__dirname, 'public')));
app.use(session({
secret: "my secret",
resave: false,
saveUninitialized: false,
store: store
}));
app.use((req, res, next) => {
console.log('This demo middleware will always run...');
console.log('req.get("host")', req.get("host"));
res.setHeader('Access-Control-Allow-Origin', '*');
res.setHeader('Access-Control-Allow-Methods', 'GET, POST, PUT');
res.setHeader('Access-Control-Allow-Headers', 'Content-Type, Authorization');
next();
});
app.use((req, res, next) => {
res.locals.isLoggedIn = req.session.isLoggedIn;
res.locals.csrfToken = req.csrfToken();
next();
});
app.use(csrfProtection); // ERROR HERE: REASON UNKNOWN
app.use(subRoutes);
app.use(authRoutes);
app.use(adminRoutes);
//app.listen(3000);
mongoConnect(() => {
app.listen(process.env.PORT || 3000);
});
console.log('App running successfully...');
Here