ExpressJs send response with error inside middleware

I can't understand why my ExpressJs app is crashing sending res.status(401) inside a middleware.

Let's say my start.js has:

app.use(middlewares.timestampValidator());

and the middleware is declared as follow:

timestampValidator: () => {

return (req, res, next) => {

    [...]
    if(error) {
        res.status(401).json(new ServerResponse());
    }
    else {
        next();
    }
}
}

When the error is -successfully- sent to the client the server crashes with this error:

node:internal/process/promises:246 triggerUncaughtException(err, true /* fromPromise */); ^

[UnhandledPromiseRejection: This error originated either by throwing inside of an async > function without a catch block, or by rejecting a promise which was not handled with > > .catch(). The promise rejected with the reason "false".] { code: 'ERR_UNHANDLED_REJECTION' }

But the functions is not async. I tried calling next('error'); after sending status 401 but the app continues to routes and then the response can't be send to client because already sent.

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