Node.js Express.js not always sending valid json

Here is the express code

app.post('/v1/sessions' function(req,res){
    res.send({id:1234});
});

For some reason the json response comes back like this

OK{ id: 1234}

Why is the OK there?

EDIT

Ok so here is all of my code. I don't see where it would be sending the OK.

var express = require('express');
var app = express();

app.enable('trust proxy');
app.use(express.bodyParser());

app.all('*', function(req, res, next) {
  res.header("Access-Control-Allow-Origin", "*");
  res.header("Access-Control-Allow-Headers", "X-Requested-With");
  next();
});

app.post('/v1/sessions', function(req, res) {
  if(req.body.email == 'testuser@captusr.com' && req.body.password == 'testpass'){
    res.send(200, JSON.stringify({token:{id:'longstring',email:'testuser@captusr.com'}}));
  } else {
    res.send({code:403, error:"Invalid email or password"});
  }
});

app.all('*', function(req, res, next) {
  res.header('Access-Control-Allow-Origin', '*');
  res.header('Access-Control-Allow-Methods', 'HEAD, GET, POST, PUT, DELETE, OPTIONS');
  res.header('Access-Control-Allow-Headers', 'Content-Type, X-Requested-With, Origin, Accept');
  res.header('Access-Control-Allow-Credentials', 'true');

  if (req.method.toLowerCase() === 'options') {
    res.send(200);
  } else {
    next();
  }
});

app.listen(3000);
console.log('Listening on port 3000');