Spawned phantomjs process hanging

I'm trying to create a node server that spawns phantomjs processes to create screenshots. The grab.js script works fine when executed and I've confirmed that it writes to stdout. Problem is the node code that spawns the process simply hangs. I've confirmed that phantomjs is in the path. Anyone know what might be happening here or how I might troubleshoot this?

Here's the phantomjs code (grab.js) that renders the page and writes the data to stdout:

var page = require('webpage').create(),
  system = require('system'),
  fs = require('fs');

var url = system.args[1] || 'google.com';

page.viewportSize = {
  width: 1024,
  height: 1200
};

page.open(url, function() {
  var b64 = page.renderBase64('png');
  fs.write('/dev/stdout', b64, 'w');
  phantom.exit();
});

And here's the node code that spawns the phantom progress and prints the result (hangs):

var http = require('http'),
  exec = require('child_process').exec,
  fs = require('fs');

exec('phantomjs grab.js google.com', function(error, stdout, stderr) {
  console.log(error, stdout, stderr);
});