multi-threading error in minimal script

[Skaperen]

i am getting the error shown below in a minimal little script that runs just 2 threads:

Output:
lt2a/forums /home/forums 1> cat -n trytwocommands.py
     1	#!/usr/bin/env python3
     2	import subprocess,threading
     3	one = threading.Thread(target=subprocess.call,args=['sleep','10001'])
     4	two = threading.Thread(target=subprocess.call,args=['sleep','10002'])
     5	one.start()
     6	two.start()
     7	print('started 2 threads, now joining them',flush=1)
     8	one.join()
     9	two.join()
    10	print('joined 2 threads, now done',flush=1)
lt2a/forums /home/forums 2> py3 trytwocommands.py
started 2 threads, now joining them
Exception in thread Thread-2:
Traceback (most recent call last):
  File "/usr/lib/python3.6/threading.py", line 916, in _bootstrap_inner
    self.run()
  File "/usr/lib/python3.6/threading.py", line 864, in run
    self._target(*self._args, **self._kwargs)
  File "/usr/lib/python3.6/subprocess.py", line 287, in call
    with Popen(*popenargs, **kwargs) as p:
  File "/usr/lib/python3.6/subprocess.py", line 629, in __init__
    raise TypeError("bufsize must be an integer")
TypeError: bufsize must be an integer

Exception in thread Thread-1:
Traceback (most recent call last):
  File "/usr/lib/python3.6/threading.py", line 916, in _bootstrap_inner
    self.run()
  File "/usr/lib/python3.6/threading.py", line 864, in run
    self._target(*self._args, **self._kwargs)
  File "/usr/lib/python3.6/subprocess.py", line 287, in call
    with Popen(*popenargs, **kwargs) as p:
  File "/usr/lib/python3.6/subprocess.py", line 629, in __init__
    raise TypeError("bufsize must be an integer")
TypeError: bufsize must be an integer

joined 2 threads, now done
lt2a/forums /home/forums 3>

i presume the 2 errors happen in the 2 threads, especially considering the output from the print calls before and after.
what is bufsize and what set it to a non-integer? is there a way i can set it? or is subprocess.call() not thread-safe?