Warning: Unable to read "undefined" file (Error code: ENOENT) when compiling coffeescript from watch

First, thank you for putting this plugin together.  I spent a good portion of my day trying various approaches to having grunt coffee compile only changed files and then livereloading those changes.

With that being said, I am running into the following error (with verbose output):

```
Running tasks: newer:coffee:dist

Running "newer:coffee:dist" (newer) task
Files: app/application.coffee -> .tmp/application.js
Files: app/boot.coffee -> .tmp/boot.js
Files: app/bower_components/backbone/test/model.coffee -> .tmp/bower_components/backbone/test/model.js
Files: app/config.coffee -> .tmp/config.js
Files: app/controllers/base/authenticated.coffee -> .tmp/controllers/base/authenticated.js
Files: app/controllers/base/controller.coffee -> .tmp/controllers/base/controller.js
Files: app/controllers/home_controller.coffee -> .tmp/controllers/home_controller.js
Files: app/controllers/session_controller.coffee -> .tmp/controllers/session_controller.js
Files: app/lib/GCS.coffee -> .tmp/lib/GCS.js
Files: app/models/base/grid_model.coffee -> .tmp/models/base/grid_model.js
Files: app/models/connect.coffee -> .tmp/models/connect.js
Files: app/models/user.coffee -> .tmp/models/user.js
Files: app/routes.coffee -> .tmp/routes.js
Files: app/vendor.coffee -> .tmp/vendor.js
Files: app/views/base/view.coffee -> .tmp/views/base/view.js
Files: app/views/home/main.coffee -> .tmp/views/home/main.js
Files: app/views/session/signin.coffee -> .tmp/views/session/signin.js

Running "coffee:dist" (coffee) task
Verifying property coffee.dist exists in config...OK
Files: [no src] -> .tmp/application.js
Files: [no src] -> .tmp/boot.js
Files: [no src] -> .tmp/bower_components/backbone/test/model.js
Files: [no src] -> .tmp/config.js
Files: [no src] -> .tmp/controllers/base/authenticated.js
Files: [no src] -> .tmp/controllers/base/controller.js
Files: [no src] -> .tmp/controllers/home_controller.js
Files: [no src] -> .tmp/controllers/session_controller.js
Files: [no src] -> .tmp/lib/GCS.js
Files: [no src] -> .tmp/models/base/grid_model.js
Files: [no src] -> .tmp/models/connect.js
Files: app/models/user.coffee -> .tmp/models/user.js
Files: [no src] -> .tmp/routes.js
Files: [no src] -> .tmp/vendor.js
Files: [no src] -> .tmp/views/base/view.js
Files: [no src] -> .tmp/views/home/main.js
Files: [no src] -> .tmp/views/session/signin.js
Options: bare, join=false, sourceMap, separator="\n", sourceRoot=""
Reading undefined...ERROR
Warning: Unable to read "undefined" file (Error code: ENOENT). Use --force to continue.
```

And the relevant portion of my Gruntfile.coffee:

```coffeescript
# monitor for changes
  watch:
    coffee:
        files: 'app/**/*.coffee'
        tasks: ['newer:coffee:dist']
```

Works if I use `any-newer` but then it is compiling every file.  Am I simply misinterpreting the docs?