Redmine

In the REST-API-Wiki you are offering parent_issue_id to use for issue creation.
I m trying to use this value to filter all issues of a project with a get-request with:

• project_id
• tracker_id
• status_id
• parent_issue_id

I want to get rid of subtickets but it s just ignoring my additions.

My request looks like:

/redmine/issues.json?project_id=pr0123&tracker_id=4&status_id=1&parent_issue_id=!*&limit=100&key=4240ba4e3andsoon

Am i just to stupid to use the parent_issue_id or is this feature not included?
Using <preparent_issue_id = !* for getting rid of subtickets would be great.