threadpool worker return empty string fails with SIGSEGV in gc

[onde]

Dear all,

I'm an excited, but rather new user of Nim and recently tried using threadpool 
to process data (read from a file line by line) in parallel with `spawn`. 
Weirdly enough the code failed during runtime with

> gc.nim(198) nimGCref
> 
> gc.nim(191) incRef
> 
> SIGSEGV: Illegal storage access. (Attempt to read from nil?)

I constructed a minimal working example that shows it fails only, if the worker 
returns an empty string (which in my case is valid). 
    
    
    import threadpool
    #{.experimental: "parallel".}
    
    proc doStuff(plp: string): string {.noSideEffect.} =
      #return "works"
      return ""# doesn't work
    
    proc call*(fName: string, maxThreads = 1) =
      var fh: File = if fName == "-": stdin else: open(fName)
      defer:
        if fh != stdin:
          fh.close
      setMaxPoolSize(maxThreads)
      for line in fh.lines:
        discard spawn doStuff(line)
    
    when isMainModule:
      import cligen
      dispatch(call)
    
    
    Run

This is obviously utterly artificial now and doesn't do anything at all, but if 
it's enough to trigger the GC SIGSEV. As soon as you return anything but an 
empty string, the example works just fine.

Could anyone give me some pointers as to what's going on?

Thanks a lot, Andreas

PS: I'm using Nim Compiler Version 0.19.0 [MacOSX: amd64]