express res.send as result of promise

I'm not understanding what's going on...

this works:

router.get('/items', (req, res) => {
    myPromise.then((result) => res.send(result));
});

but this doesn't, it keeps the browser like if the request never ends:

router.get('/items', (req, res) => {
    myPromise.then(res.send);
});

What's wrong?