[IronPython] [Newbie] Problem with Getting Started code

[Michael M Mason]

I've been trying the Getting Started example code (from
http://ironpython.net/browser/gettingstarted.html) but I get
an error message. The message is:-

  TypeError: unsupported operand type(s) for +: 'NoneType' and
'function'

and the code that causes it is this (marked line appears to be
the cause of the error):-

    <input id="button" type="button" value="Junk 2 - Say Hello!" />
    <script type="text/python">
    def button_onclick(s, e):
        window.Alert("Hello from Python!")
--> document.button.onclick += button_onclick
    </script>

Can anyone explain what the correct code would be?

-- 
Michael