Multi-methods don't work when passing ref to a different thread

[endragor]

Sample:

import tables

type
  Base* = ref object of RootObj
  Derived* = ref object of Base

method someMethod(self: Base) =
  echo "Base"
method someMethod(self: Derived) =
  echo "Derived"

type
  ThreadPool = ref object
    threads: seq[ptr Thread[ptr TableChannel]]
  TableChannel = Channel[TableRef[string, Base]]

var globalTable {.threadvar.}: TableRef[string, Base]
globalTable = newTable[string, Base]()
globalTable.add("obj", new(Derived))

proc testThread(channel: ptr TableChannel) {.thread.} =
  globalTable = channel[].recv()
  globalTable["obj"].someMethod()

proc newThreadPool(threadCount: int): ThreadPool =
  new(result)
  result.threads = newSeq[ptr Thread[ptr TableChannel]](threadCount)
  for i in 0..<threadCount:
    var channel = cast[ptr TableChannel](allocShared0(sizeof(TableChannel)))
    channel[].open()
    var threadPtr = cast[ptr Thread[ptr TableChannel]](alloc0(sizeof(Thread[ptr TableChannel])))
    createThread(threadPtr[], testThread, channel)
    result.threads[i] = threadPtr
    channel[].send(globalTable)

discard newThreadPool(8)

This code should print Derived 8 times, but nothing gets printed, even though someMethod invocation is successful (no crash or anything). It looks as if nothing at all gets invoked.