Thread-safe version of mock.call_count

It appears that Mock.call_count does not work correctly with threads. For instance:

import threading
import time
from mock import MagicMock


def f():
    time.sleep(0.1)

def test_1():
    mock = MagicMock(side_effect=f)
    nb_threads = 100000
    threads = []
    for _ in range(nb_threads):
        thread = threading.Thread(target=mock)
        threads.append(thread)
        thread.start()

    for thread in threads:
        thread.join()

    assert mock.call_count == nb_threads, mock.call_count

test_1()

This code produced the following output:

Traceback (most recent call last):
  File "test1.py", line 24, in <module>
    test_1()
  File "test1.py", line 21, in test_1
    assert mock.call_count == nb_threads, mock.call_count
AssertionError: 99994

Is there a way I can use call_count (or similar) within a multithreaded portion of code? I'd like to avoid having to rewrite MagicMock myself...