How to retrieve the audiences for a given campaign via the Google Ads API

I'm trying to fetch the audiences (Custom intent, Custom affinity) of all my enabled google display campaigns.

I thought I can get it through the CampaignCriterionService, but the returned data is empty.

    campaign_criterion_service = ADWORDS_CLIENT.GetService(
        'CampaignCriterionService', version='v201809')

    selector = {
        'fields': ['CampaignId', 'Id', 'CriteriaType', "PlacementUrl",
                   'KeywordText', "ChannelName", "AppId", "DisplayName", "CampaignStatus", "CampaignName"],
        'predicates': [{
            'field': 'CriteriaType',
            'operator': 'IN',
            'values': [
                'CUSTOM_AFFINITY',
                'CUSTOM_INTENT'
            ]
        },
        {
            'field': 'CampaignStatus',
            'operator': 'IN',
            'values': [
                'ENABLED'
            ]
        }


        ],
        'paging': {
            'startIndex': 0,
            'numberResults': 500
        }
    }

    page = campaign_criterion_service.get(selector)

Any clues?

from How to retrieve the audiences for a given campaign via the Google Ads API