Why I can't get “merge requests reviewers” from “Get single MR” API(CE 13.5.5)?

Excuse me, I used gitlab API for the first time. I want to get MR reviewer. From API docs, I find "Get single MR" can be realised. But there is no return reviewer value when I actually use the API. Why is it so? Thanks! My gitlab version is CE 13.5.5