Promise not resolving - Mocha + CasperJS + Q

Question

I'm trying to use the Q promise library in my mocha-casperjs tests. I'm wondering if anyone could help me understand why the code below won't resolve? The code is written in coffeescript and launched using mocha-casperjs

Q = require('./q')
casper.errorHandler = ->
    @echo @ 
    throw new CasperError 'Promise Failed'

## Promise version of casper.open()
casper._open = (url) ->
    deferred = Q.defer()
    @open url, deferred.resolve
    return deferred.promise

describe 'Main Test', ->
    before ->
        casper.start()
        console.log 'casper started'
    it 'Test Case 1', (done) ->
        casper._open('http://google.com')
        .then ( ->
            'Google'.should.matchTitle
            done()), casper.errorHandler()

When the error handler's called, casper echoes [object Casper], currently at about:blank which means the page hasn't loaded yet. But I thought the promise function would wait for that before moving onto the .then()?