YouTube Data API - Empty "items" list from Activities:list

I'm trying to get user activity (liking, subscribing, etc.) through YouTube's Data API. However, when I send a GET request here: https://youtube.googleapis.com/youtube/v3/activities?part=snippet%2CcontentDetails&maxResults=25&mine=true&key=[MY API KEY]

I get a 200 response but with the "items" empty:

{"kind": "youtube#activityListResponse","etag": "Gt78OtFG_qGDXWi8-LJKjdLhoCY","items": [],"pageInfo": {"totalResults": 2,"resultsPerPage": 25  }}

I know that the account I linked it to has activity, so I'm confused by it's not returning any information. Any assistance would be appreciated.