As documented startFrom parameter should be a string, but cmstartsortkey and cmendsortkey in API are for binary keys like '5cba0a9424348eceba340307045c0a6cba347cff5d330115018f7c8f0900'.
Maybe API was changed.
I think it's need to change in _parseCategory:
cmstartsortkey -> cmstartsortkeyprefix
cmendsortkey -> cmendsortkeyprefix
Version: compat-(1.0)
Severity: normal