po("threshold") cannot set 'prob'

[loveyu3317]

I want to tune threshold by classif.auc. But when I run the following codes, it cannot set predict_type = "prob".

gr = lrn("classif.rpart", predict_type = "prob") %>>% po("threshold") 
l = as_learner(gr) 
l$predict_type 

[loveyu3317]

And when I run :

learner$predict_type = 'prob'

it says :

Error in .__PipeOpThreshold__predict_type(self = self, private = private,  : 
  $predict_type for PipeOpThreshold is read-only.