Typescript Error: Property 'user' does not exist on type 'Request'

I have the following piece of code in my express app

router.get('/auth/userInfo', this.validateUser,  (req, res) => {
    res.json(req.user);
});

and my IDE seems to be complaining with the error

error TS2339: Property 'user' does not exist on type 'Request'.

When I compile my typescript code it seems to be throwing this error. Any ideas why this is happening?

via RRP