Abstract type must resolve to an Object type at runtime for field Query error

Start by running the provided schema.graphql example through graphql.build_ast_schema and graphql.graphql, then inspect runtime resolution for the Address union returned by Query.resolve_address. Done means the query completes without the abstract-type error and produces the expected L2/L3 data.

I am trying to use unions and see the above error. Any help appreciated. Here's the schema:

schema {
     query: Query
}
union Address = L3 | L2
type L2 {
  mac: String
}
type L3 {
  ip: String
}
type Query {
  address(t: String): Address
}

Here's the example code:

import graphql, sys, json

class Query(object):
    def resolve_address(self, args, context, info):
        typ = args.get('t')
        if  typ == "L3":
          return '10.1.1.10'
        else:
          return '00:aa:bb:cc:dd:ee'
######################################################################

def resolve_fn(obj, args, context, info):
    # If obj is a mapping, return obj[field_name], else if
    # obj.resolve_field_name() exists call it, else return obj.field_name.
    field_name = info.field_name
    if hasattr(obj, '__getitem__'):
        return obj[field_name]
    rfn = getattr(obj, "resolve_%s" % field_name, None)
    if rfn and callable(rfn):
        return rfn(args, context, info)
    else:
        return getattr(obj, field_name)

class MiddlewareManager(graphql.MiddlewareManager):
    def get_field_resolver(self, field_resolver):
        return resolve_fn
######################################################################

query = '{address(t:"L2"){... on L3{ip} ...on L2{mac}}}'

schema_source = open('schema.graphql').read()
schema = graphql.build_ast_schema(graphql.parse(schema_source))

r = graphql.graphql(schema, query,
                    root_value=Query(),
                    context_value=123,
                    middleware=MiddlewareManager())
print "errors:", r.errors
print "data:", r.data
print "json:", json.dumps(r.data)

Similar example written in Graphene works fine but I would like to use schema written in native grpahql format.