How to use an array in Pony?

Question

Since the documentation and tutorials on collections in Ponylang are lacking, I'm really struggling in using the Array class and working with indices.

I have an overenginered actor-based fizzbuzz where there's an actor in charge of requesting fizzbuzz operations to other actors and collecting the results. Probably the pattern I'm using is not the best but right now I would like to iterate in this way to learn more.

My problem is with the following code:

actor FizzBuzzer
    var _results:Array[String] 
    let _main: Main tag


fun list_to_string(l:List[String]):String=>
   l.fold[String]({(a:String,b:String)=>a+"\n"+b},"")

new create(n:String, main:Main tag)=>

  let num:USize = try (consume n).usize()? else 0 end
  _main = main
  _results = recover Array[String] end
  try

    let result = this.fizzbuzz(num)?
    let message:String = list_to_string(result)
    main.print(message)
  else
    main.print("Invalid argument: "+ num.string())
  end

fun fizzbuzz(n:USize,  acc:List[String]=List[String]()):List[String] ?=>
  if n <=0 then error end

  let res = List[String]()
  for i in Range[USize](1,n+1) do
     FizzBuzzerino.process(i,this)
  end
  res 

be collect_result(result:String,num:USize)=>
     try
        _results.insert(num,result)?
      else
        _main.print("Error processing element: "+num.string())
      end
     `

In the function collect_result the insertion always fails. Using update is the same. The result and num I receive from the processing actor are correct but I cannot insert them in the array. Any insertion in the Array in this class fails. Is it a matter of capabilities? Or am I doing something wrong?