UnknownAttributeError 'remove_source_branch' when closing merge request

When closing a merge request and the remove_source_branch variable is part of the body, the server responds with a 500 Internal Server Error.

Response body:

{"message":"500 Internal Server Error"}

According to the docs (https://docs.gitlab.com/ce/api/merge_requests.html#update-mr), this should be valid.

The logs spits out a 'ActiveModel::UnknownAttributeError (unknown attribute 'remove_source_branch' for MergeRequest.)'

curl -v --header "PRIVATE-TOKEN:<token>" \
  --header "Content-Type: application/json" \
  --request PUT \
  --data '{"id":"<id>","iid":"<iid>", "state_event" : "close", "remove_source_branch": false}' \
  https://<host>/api/v4/projects/<id>/merge_requests/<iid>

Nevertheless, the merge requests is closed.

GitLab CE version: 11.9.6