Flask-Assets KeyError

gistfile1.txt

raceback (most recent call last):
  File "/Users/olt/dev/venv/foo/lib/python2.7/site-packages/flask/app.py", line 1518, in __call__
    return self.wsgi_app(environ, start_response)
  File "/Users/olt/dev/venv/foo/lib/python2.7/site-packages/flask/app.py", line 1506, in wsgi_app
    response = self.make_response(self.handle_exception(e))
  File "/Users/olt/dev/venv/foo/lib/python2.7/site-packages/flask/app.py", line 1504, in wsgi_app
    response = self.full_dispatch_request()
  File "/Users/olt/dev/venv/foo/lib/python2.7/site-packages/flask/app.py", line 1264, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/Users/olt/dev/venv/foo/lib/python2.7/site-packages/flask/app.py", line 1262, in full_dispatch_request
    rv = self.dispatch_request()
  File "/Users/olt/dev/venv/foo/lib/python2.7/site-packages/flask/app.py", line 1248, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/Users/olt/dev/foo/experiments/assets/test_bundles.py", line 14, in hello
    return render_template('test_bundles.html')
  File "/Users/olt/dev/venv/foo/lib/python2.7/site-packages/flask/templating.py", line 123, in render_template
    context, ctx.app)
  File "/Users/olt/dev/venv/foo/lib/python2.7/site-packages/flask/templating.py", line 107, in _render
    rv = template.render(context)
  File "/Users/olt/dev/venv/foo/lib/python2.7/site-packages/jinja2/environment.py", line 894, in render
    return self.environment.handle_exception(exc_info, True)
  File "/Users/olt/dev/foo/experiments/assets/templates/test_bundles.html", line 1, in top-level template code
    {% assets "js-all" %}
  File "/Users/olt/dev/venv/foo/lib/python2.7/site-packages/webassets/ext/jinja2.py", line 102, in _render_assets
    'filters': filter}).urls(env=env)
  File "/Users/olt/dev/venv/foo/lib/python2.7/site-packages/webassets/bundle.py", line 448, in urls
    urls.extend(bundle._urls(env, extra_filters, *args, **kwargs))
  File "/Users/olt/dev/venv/foo/lib/python2.7/site-packages/webassets/bundle.py", line 421, in _urls
    *args, **kwargs)
  File "/Users/olt/dev/venv/foo/lib/python2.7/site-packages/webassets/bundle.py", line 329, in _build
    elif not path.exists(env.abspath(self.output)):
  File "/Users/olt/dev/venv/foo/lib/python2.7/site-packages/webassets/env.py", line 289, in abspath
    return path.abspath(path.join(self.directory, filename))
  File "/Users/olt/dev/venv/foo/lib/python2.7/site-packages/webassets/env.py", line 260, in get_directory
    return self.config['directory']
  File "/Users/olt/dev/venv/foo/lib/python2.7/site-packages/flaskext/assets.py", line 70, in __getitem__
    raise KeyError()
KeyError

test.py

from flask import Flask, render_template
from flaskext.assets import Environment, Bundle

app = Flask(__name__, static_path='/foo')
assets = Environment(app)

js = Bundle('jquery.js', 'base.js', 'widgets.js',
            filters='jsmin', output='gen/packed.js')
assets.register('js-all', js)


@app.route("/")
def hello():
    return render_template('test_bundles.html')

if __name__ == "__main__":
    app.debug = True
    app.run()

test_bundles.html

{% assets "js-all" %}
    <script type="text/javascript" src="{{ ASSET_URL }}"></script>
{% endassets %}