AttributeError:'google.protobuf.pyext._message.RepeatedCompositeCo' object has no attribute 'DESCRIPTOR'

def get_ad_full_details(ad_json, current_topics, ad_group_ad, ad):
        mandatory_data = {
            "ad_group_ad.ad.responsive_search_ad.headlines": ad_group_ad.ad.responsive_search_ad.headlines,
            "ad_group_ad.ad.responsive_search_ad.descriptions": ad_group_ad.ad.responsive_search_ad.descriptions}
    ad_json["mandatory_data"] = mandatory_data

when I run json.dumps(ad_json) I get AttributeError:'google.protobuf.pyext._message.RepeatedCompositeCo' object has no attribute 'DESCRIPTOR'. I've tried to follow this post but it still gives the same error.

I've tried to iterate and map the ad_group_ad.ad.responsive_search_ad.headlines repeated protof field to its "[text]" values, but the code fails.

Any idea, how I can fetch the "text" member of this repeated proto field?

I can use regex, but thought their might be an easier way