Unable to serve index.html at the route "/"

The following doesn't work, returning a file not found error.

@app.route("/")
def root(request):
   return File("index.html")

The following works

@app.route("/index.html")
def root(request):
   return File("index.html")

And this also works

@app.route("/")
def root(request):
   return "Hello"

This is a failing test that I think illustrates the problem

    def test_staticRoot_noBranch(self):
        """
        The following should return the contents of the file index.html

        @app.route("/")
        def root(request):
            return File("index.html")

        """
        app = self.app
        request = requestMock("/")

        @app.route("/")
        def root(request):
            return File(__file__)

        d = _render(self.kr, request)

        def _cb(result):
            self.assertEqual(request.getWrittenData(), 
                open(__file__).read())
            self.assertEqual(request.finishCount, 1)

        d.addCallback(_cb)
        return d

Start with the failing test_staticRoot_noBranch test and trace the _render path for a request to "/". Compare it with the working "/index.html" and string-return cases shown in the issue. Done means the test passes and serving File(file) at "/" writes the file contents and finishes the request once.