Parallel statement: procedure returning string

[onde]

Dear all,

I would like to use 
[parallel](https://nim-lang.org/docs/manual_experimental.html#parallel-amp-spawn-parallel-statement)
 and am running into unexpected compilation problems, apparently because the 
procedure in question returns a string. An artificial example is given below. 
For this I took the normally perfectly fine working code (if it returns a 
float) for efficiently computing pi from the 
[docs](https://nim-lang.org/docs/manual_experimental.html#parallel-amp-spawn-parallel-statement)
 and made it return a string:
    
    
    import strutils, math, threadpool
    {.experimental: "parallel".}
    
    proc term(k: float): string =
      var f = 4 * math.pow(-1, k) / (2*k + 1)
      result = $f
    
    proc pi(n: int): float =
      var ch = newSeq[string](n+1)
      parallel:
        for k in 0..ch.high:
          ch[k] = spawn term(float(k))
      #for k in 0..ch.high:
      #  result += ch[k]
      result = ch[0]
    
    echo formatFloat(pi(5000))

This obviously doesn't do anything, but it does trigger the compilation error:
    
    
    Error: type mismatch: got <FlowVar[system.string]> but expected 
'TaintedString = string'

I don't quite understand what's going on. Using TaintedString and switching 
taintMode on or not, doesn't change anything.

Could someone please help?

Many thanks, Andreas