[API] update_plan_entry throws error

I am using .Net library at https://github.com/gurock/testrail-api/ … master.zip.

API doco -

POST update_plan_entry/:plan_id/:entry_id
{
  name: string,
  assignedto_id: int,
  include_all: bool,
  case_ids: array
}

My request -
POST update_plan_entry/13/23
{
  include_all: false,
  case_ids: [9, 171]
}

All ids are correct and proper for the request.

The response -

{"TestRail API returned HTTP 400 (\"Field :entry_id is not a valid test plan entry.\")"}