getSeries() on an non-existing raises BadRequest, would expect NotFound

>>> from launchpadlib.launchpad import Launchpad
>>> lp = Launchpad.login_with('test_case')
>>> lp.distributions['ubuntu'].getSeries(name_or_version='foobar')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/francis/canonical/launchpad/devel/eggs/lazr.restfulclient-0.11.2-py2.6.egg/lazr/restfulclient/resource.py", line 573, in __call__
    url, in_representation, http_method, extra_headers=extra_headers)
  File "/home/francis/canonical/launchpad/devel/eggs/lazr.restfulclient-0.11.2-py2.6.egg/lazr/restfulclient/_browser.py", line 281, in _request
    raise error
lazr.restfulclient.errors.BadRequest: HTTP Error 400: Bad Request
Response headers:
---
connection: close
content-length: 38
content-type: text/plain
date: Wed, 15 Jun 2011 20:58:03 GMT
server: zope.server.http (HTTP)
status: 400
vary: Accept,Accept-Encoding
via: 1.1 wildcard.staging.launchpad.net
x-lazr-notifications: []
x-powered-by: Zope (www.zope.org), Python (www.python.org)
---
Response body:
---
No such distribution series: 'foobar'.
---

I would have expected a NotFound (404) error.