I hesitate to call this a bug because, as advertised, it does work for "-1" spell point cost. But, if you have anything other than "-1" it will not reduce the used pp total if there are less used points than the absolute value of the negative spell cost. (e.g. the spell cost is -2 and points used is 1, the used won't drop to 0 it will stay at 1).
Very cool little extension though. Thank you!