ConditionManagement10.CompareConditionCategories not working

I'm curious as to whether anyone out there has managed to get any output out of this ancient API call? I've gotten output out of every other call to this API, but the response object for this call always comes back with a return code of -1 and an empty payload. No exception is thrown.

I'm hoping this call works correctly as I'm not looking forward to rolling my own categories XML comparison algorithm :(

Many thanks for any help you can give.