Node.js and Sendgrid mailer error on res.end

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?