Node.js and Sendgrid mailer error on res.end - node.js

Question

I'm a bit new to node. I'm using express and the sendgrid api to send an email (collected REST-fully). After sendgrid succeeds or fails, I want to respond with a json object. Here's the sample case:
var SendGrid = require('sendgrid-nodejs').SendGrid;
var sendgrid = new SendGrid(user, key);
app.get('/LGP/:email', function (req, res){
sendgrid.send({
to: req.params.email,
from: 'me#example.com',
subject: 'Hello World',
text: 'This email sent through SendGrid'
}, function(success, message) {
if (!success) {
console.log(message);
} else {
res.writeHead(200, { 'Content-Type': 'application/json' });
res.write(JSON.stringify({ result: 'success' }));
res.end(); //error occurs: "Can't use mutable header APIs after sent."
}
}
);
});
On my local server (using foreman), everything works fine. But when I push it to heroku, it gives me this stack trace:
2013-02-27T22:12:46+00:00 app[web.1]: http.js:543
2013-02-27T22:12:46+00:00 app[web.1]: throw new Error("Can't use mutable header APIs after sent.");
2013-02-27T22:12:46+00:00 app[web.1]: ^
2013-02-27T22:12:46+00:00 app[web.1]: Error: Can't use mutable header APIs after sent.
2013-02-27T22:12:46+00:00 app[web.1]: at ServerResponse.getHeader (http.js:543:11)
2013-02-27T22:12:46+00:00 app[web.1]: at /app/node_modules/express/node_modules/connect/lib/middleware/logger.js:229:26
2013-02-27T22:12:46+00:00 app[web.1]: at ServerResponse.<anonymous> (/app/node_modules/express/node_modules/connect/lib/middleware/logger.js:149:20)
2013-02-27T22:12:46+00:00 app[web.1]: at /app/app.js:60:13
2013-02-27T22:12:46+00:00 app[web.1]: at IncomingMessage.<anonymous> (/app/node_modules/sendgrid/lib/sendgrid.js:74:9)
2013-02-27T22:12:46+00:00 app[web.1]: at IncomingMessage.emit (events.js:81:20)
2013-02-27T22:12:46+00:00 app[web.1]: at HTTPParser.onMessageComplete (http.js:133:23)
2013-02-27T22:12:46+00:00 app[web.1]: at CleartextStream.ondata (http.js:1213:22)
2013-02-27T22:12:46+00:00 app[web.1]: at CleartextStream._push (tls.js:291:27)
2013-02-27T22:12:46+00:00 app[web.1]: at SecurePair._cycle (tls.js:565:20)
2013-02-27T22:12:48+00:00 heroku[web.1]: Process exited with status 1
2013-02-27T22:12:48+00:00 heroku[web.1]: State changed from up to crashed
/app/app.js:60:13 refers to the line with "res.end()". What am I doing wrong?

Answer 1

To maximize your chances of your app behaving the same locally and on heroku, you should make sure you specify specific versions for all modules and avoid using "*" for the main dependencies.
Should also should specify the node version in package.json:
"engines": {
"node": "0.8.20"
}
(use whatever version is appropriate).