Why is my custom JSONEncoder.default() ignoring booleans?

I want to convert a dictionary to a JSON string with boolean True values translated to the number 1 and boolean False values translated to the number 0. I'm using a JSONEncoder subclass, but it seems to ignore booleans ...

import json

class MyEncoder(json.JSONEncoder):

    def default(self, obj):
        if isinstance(obj, bool):
            return 1 if obj else 0
    return super().default(obj)

data = { 'key-a' : 'a', 'key-true' : True, 'key-false' : False }

jsondata = json.dumps(data, cls=MyEncoder)

print(jsondata)

I want this to be the result:

{"key-true": 1, "key-a": "a", "key-false": 0}

However, this is what I get:

{"key-true": true, "key-a": "a", "key-false": false}

I know I can programatically modify the data before passing it to json.dumps, but is there any way I can obtain my desired result via a JSONEncoder subclass?