New Post: access indexer of a hosted object

I am trying to get my NamedNodeMap type working which may be indexed either by a number or a string. If I implement IList then I can evaluate expressions like map[0] -> Attr(href="") but map['href'] = undefined. If I implement/inherit either IPropertyBag or DynamicObject then map['href'] acts as expected but map[0] = undefined.

Am I missing something obvious? I tried using DynamicObject's overrides for getting/setting members by name and index.