async in a thread; crashes

[enthus1ast]

i'm fighting with an async in a thread example, does one have a tip where my 
big mistake is? It works for a few http request then crashes, it crashes faster 
in release, but not crashes withouth gc (or not discovered yet...)
    
    
    import locks
    import os
    import asyncdispatch
    import asynchttpserver, asyncdispatch
    
    var L: Lock
    
    type Context = ref object {.pure.}
      num: int
      str: string
    
    proc th(ctxP: pointer) {.thread.} =
      var server = newAsyncHttpServer()
      proc cb(req: Request) {.async.} =
        var idx: int = 0
        if not tryAcquire(L): return
        var ctx = cast[ref Context](ctxP)
        GC_ref ctx # ????
        ctx[].num.inc()
        idx = ctx[].num
        GC_unref ctx # ????
        release(L)
        await req.respond(Http200, "Hello World " & $idx)
      proc aprint(): Future[void] {.async.} =
        while true:
          echo "hello"
          await sleepAsync 5000
      asyncCheck aprint()
      asyncCheck server.serve(Port(8080), cb)
      runForever()
    
    initLock(L)
    var ctx = Context(num: 0, str: "")
    var ctxP = ctx.addr
    GC_ref ctx # ????
    var networkThread: Thread[pointer]
    networkThread.createThread(th, ctxP)
    while true:
      if not tryAcquire(L):
        echo "could not aquire lock"
        sleep(50)
        continue
      ctx.num.inc
      echo(ctx.num)
      release(L)
      sleep(2500)
    
    
    Run

it crashes with:
    
    
    10004
    10263
    hello
    Traceback (most recent call last)
    threadAsync.nim(30)      th
    asyncdispatch.nim(1651)  runForever
    asyncdispatch.nim(1516)  poll
    asyncdispatch.nim(1282)  runOnce
    asyncdispatch.nim(191)   processPendingCallbacks
    asyncmacro.nim(36)       serve_continue
    asynchttpserver.nim(309) serveIter
    asyncmacro.nim(307)      processClient
    asyncmacro.nim(36)       processClient_continue
    asynchttpserver.nim(281) processClientIter
    asyncfutures.nim(77)     newFutureVar
    asyncfutures.nim(59)     newFuture
    strmantle.nim(63)        nimIntToStr
    gc.nim(475)              newObj
    SIGSEGV: Illegal storage access. (Attempt to read from nil?)
    Error: execution of an external program failed: 
'/home/david/nimPlayground/threadAsync '
    
    
    Run

I've stressed it with:
    
    
     true; do curl 127.0.0.1:8080; done
    
    Run