Github API List repository issues missing closed_by in response

When I use the free version of the Github API,the List repository issues interface missing closed_by in response, but the interface doc has this field;

api is here : 议题 - GitHub Docs

this is my request: /repos/warmliX/OnlineJudge/issues?state=all&&since=2019-08-27T00:00:00%2B0800&&per_page=100&&page=1