Commits diff api cannot get more than 100 results

there are more than 100 diff results in a commit actually, but when I use the “/projects/:id/repository/commits/:sha/diff” api to get the diff results, it just returned only 100 results.
I try to add “per_page” and “page” parameters to get other results as following, but it return null.


the X-Total-Pages in response header is also 1.

gitlab version is GitLab Community Edition 12.3.5

how could I get all the diff results?
many thanks in advance!