OAuth2 with Node, Express, and Request - node.js

Question

I've been attempting to establish a simple Oauth2 flow using Node, Express, and Request, but I keep running into issues at the token request stage. In my specific example, I'm running an Oauth2 with MailChimp, but the problem persists with any platform.
In this example, after I post the code to get the token, I receive a response that tells me { error: 'invalid_request', error_description: 'Invalid grant_type parameter or parameter missing' }. That makes me think I'm not posting the data properly... I'm at the point that I'm convinced it's some core concept I'm missing in my logic. Any recommendations are appreciated.
Here's what I'm doing:
/* Redirect the user to MailChimp login */
app.get(`/login`, function (req, res) {
res.redirect(url.format({
pathname: 'https://login.mailchimp.com/oauth2/authorize/',
query: {
response_type: 'code',
client_id: client_id,
redirect_uri: 'http://127.0.0.1:3000/auth/'
}
}));
});
/* Recieve the code from MailChimp, and post it back to request a token */
app.get(`/auth`, function (req, res) {
request.post('https://login.mailchimp.com/oauth2/token', {
json: true,
gzip: true,
body: {
grant_type: 'authorization_code',
client_id: client_id,
client_secret: client_secret,
redirect_uri: 'http://127.0.0.1:3000/authed',
code: req.query.code,
},
}, function (err, response, body) {
console.log(response);
});
});
/* Success! We'll do something with the token here... */
app.post('/authed', function(req, res) {
res.send('we did it!')
});