Filter summary report in Clockify API to show only billable hours

I'm trying to use the API to retrieve only billable hours.

If "billable" is set to anything other than "BOTH", the API responds with an error.

I'm using the following endpoint:

https://api.clockify.me/api/workspaces/[workspace id]/reports/summary/

This is my request body:

{
    "archived": "Active",
    "billable": "true",
    "clientIds": [],
    "description": "",
    "endDate": "2019-05-31T23:59:59.999Z",
    "firstTime": "true",
    "includeTimeEntries": "true",
    "me": "true",
    "name": "",
    "projectIds": [],
    "startDate": "2019-01-01T00:00:00.000Z",
    "tagIds": [],
    "taskIds": [],
    "userGroupIds": [],
    "userIds": [],
    "zoomLevel": "week"
}

The error message I receive if I set "billable" to "true" (or "false" for that matter..) is:

{
    "message": "No enum constant com.clockify.domain.model.Billable.true",
    "code": 501
}

From the official docs, the available values for the property "billable" are:

"BOTH", "BILLABLE", "NOT_BILLABLE"