Incorrect "unresolved attribute reference" in dict Follow

If I define the following block:

d = {
    'prop1': defaultdict(int),
    'prop2': defaultdict(list)
}

d['prop2']['entry1'].append(1)

I am getting "Unresolved attribute reference 'append' for class 'int'" which is obviously untrue because d['prop2'] is a defaultdict(list) not int.