webapp2/wsgiref: "multiple values for keyword argument" when running on WSGI server

Question

The following code raises an error if I run it on a WSGI server, but not otherwise.

class Handler(webapp2.RequestHandler):
    def __init__(self, template, *args, **kwargs):
        print "Kwargs in Handler:", kwargs
        self.template = template
        super(Handler, self).__init__(*args, **kwargs)

class MainHandler(Handler):
    def __init__(self, *args, **kwargs):
        template = jinja_env.get_template("helloworld.html")
        print "Kwargs in MainHandler:", kwargs
        super(MainHandler, self).__init__(template= template, *args, **kwargs)

If I run it on a server, by doing

from wsgiref.simple_server import make_server
server = make_server('localhost', 8000, app);

while True:
    server.handle_request()

and trying to visit localhost:8080, I get

Kwargs in MainHandler: {}
ERROR:root:__init__() got multiple values for keyword argument 'template'
Traceback (most recent call last):
  File "/media/Data/code/kaiwarta/env/local/lib/python2.7/site-packages/webapp2.py", line 1535, in __call__
    rv = self.handle_exception(request, response, e)
  File "/media/Data/code/kaiwarta/env/local/lib/python2.7/site-packages/webapp2.py", line 1529, in __call__
    rv = self.router.dispatch(request, response)
  File "/media/Data/code/kaiwarta/env/local/lib/python2.7/site-packages/webapp2.py", line 1278, in default_dispatcher
    return route.handler_adapter(request, response)
  File "/media/Data/code/kaiwarta/env/local/lib/python2.7/site-packages/webapp2.py", line 1101, in __call__
    handler = self.handler(request, response)
  File "main.py", line 24, in __init__
    super(MainHandler, self).__init__(template= template, *args, **kwargs)
TypeError: __init__() got multiple values for keyword argument 'template'

(Note that kwargs is empty before being passed to Handler.)

But if I don't start a server and do foo = MainHandler() instead, it runs as expected:

Kwargs in MainHandler: {}
Kwargs in Handler: {}

What's going wrong in the first case?