Error reading parameter for analysis block

I continue receiving "internal BEAT system error"s while trying to access a parameter in an analysis block. The relevant code is something like:

## INPUTS               
## scores:          tutorial/probe_scores/1
## INPUTS_END

## RESULTS          
## ROC:             beat/chart/1
## RESULTS_END

## PARAMETERS
## points:          uint16 100
## PARAMETERS_END

import bob

class Algorithm:

  def __init__(self):
    self.positives = []
    self.negatives = []
    
  def setup(self, parameters):
    self.points = parameters.get('points', 100)
    return True

  def process(self, inputs, output):
    ...
    roc = bob.measure.roc(self.negatives, self.positives, self.points)
    ...
    return True

(see https://www.beat-eu.org/platform/algorithms/siebenkopf/ROC/6, if you can).