Create Database Backup Errors

[cygnusross]

The following call:

$ploi->servers($server->id)->databases($database->id)->backups()->create(
        $interval = 0,
        $type,
        $table_exclusions = null,
        $locations = '...',
        $path = null
    );

is throwing the error below:

{
  "message":"The selected type is invalid. (and 1 more error)",
  "errors":
  {
    "type":
    ["The selected type is invalid."],
    "keep_backup_amount":
    ["The keep backup amount field is required."]
  }
}

I've followed the API documentation and made a HTTP request with success, but the SDK doesn't look like it matches the API documentation.