DynamicAssets doesn't work with JavascriptAsset

[SimeonC]

If I do the following:

new rack.DynamicAssets
    type: rack.JavascriptAsset
    urlPrefix: '/assets/js'
    dirname: __dirname + '/assets/coffeescript'
    filter: '.coffee'

I get this error:

/Users/simeon/Sites/LOL Team Balancer/node_modules/async/lib/async.js:129
        if (!arr.length) {
                ^
TypeError: Cannot read property 'length' of undefined
  at Object.async.eachSeries (/Users/simeon/Sites/LOL Team Balancer/node_modules/async/lib/async.js:129:17)
  at JavascriptAsset.exports.JavascriptAsset.JavascriptAsset.create (/Users/simeon/Sites/LOL Team Balancer/node_modules/asset-rack/compiled/modules/javascript.js:41:20)
  at /Users/simeon/Sites/LOL Team Balancer/node_modules/asset-rack/compiled/asset.js:153:24
  at process._tickCallback (node.js:415:13)

As far as I can tell is that the JavascriptAsset needs an array of codes, and DynamicAssets isn't passing it in that. Have I missed something about this or should it work? I kinda was trying to avoid writing in all the files I wanted to be served as JS...