fun.approx = "neural.network" uninformative error

[berndbischl]

m = mountainCar()

preprocessState = function(state) {
  return(state[1,])
}

set.seed(123)
res = qlearning(m, fun.approx = "neural.network", 
  preprocessState = preprocessState, n.episodes = 100)

leads to:

Using TensorFlow backend.
Error in do.call(object$compile, args) :
'what' must be a character string or a function