SOLVED: Force heroku to use a specific version of Node.js

I've written a node.js app that works great locally, but when I push to heroku, I get the following error:

2012-04-11T00:42:55+00:00 app[web.1]:         throw e; // process.nextTick error, or 'error' event on first tick
2012-04-11T00:42:55+00:00 app[web.1]:         ^
2012-04-11T00:42:55+00:00 app[web.1]: TypeError: Object #<Object> has no method 'isArray'

I've figured out that heroku's Node is v0.4.7, but mine locally is v0.6.3. I'm specifying to use v0.6.3 in my package.json file:

"engine": {  
    "node": "0.6.3"  
  , "npm": "1.1.9"
}

But that's obviously not doing anything, even after I push.

This has been bothering me all day, and all I need to do is figure out how to get the util.isArray() method working again (hopefully). That's the error I'm getting, and it's because that's not supported in v0.4.7 of node.

Thank you!

EDIT: I was using "engine", not "engines" in my packgage.json file. </facepalm>