TypeError: cannot pickle 'weakref' object

[makotonakai]

When I executed the 4th code cell in the 'quantum-teleportation.ipynb', I got the following error.

TypeError                                 Traceback (most recent call last)
<ipython-input-5-6b43832d0e46> in <module>
     15 # Run everything
     16 alice.start()
---> 17 bob.start()
     18 alice.join()
     19 bob.join()

/Applications/Xcode.app/Contents/Developer/Library/Frameworks/Python3.framework/Versions/3.8/lib/python3.8/multiprocessing/process.py in start(self)
    119                'daemonic processes are not allowed to have children'
    120         _cleanup()
--> 121         self._popen = self._Popen(self)
    122         self._sentinel = self._popen.sentinel
    123         # Avoid a refcycle if the target function holds an indirect

/Applications/Xcode.app/Contents/Developer/Library/Frameworks/Python3.framework/Versions/3.8/lib/python3.8/multiprocessing/context.py in _Popen(process_obj)
    222     @staticmethod
    223     def _Popen(process_obj):
--> 224         return _default_context.get_context().Process._Popen(process_obj)
    225 
    226 class DefaultContext(BaseContext):

/Applications/Xcode.app/Contents/Developer/Library/Frameworks/Python3.framework/Versions/3.8/lib/python3.8/multiprocessing/context.py in _Popen(process_obj)
    281         def _Popen(process_obj):
    282             from .popen_spawn_posix import Popen
--> 283             return Popen(process_obj)
    284 
    285     class ForkServerProcess(process.BaseProcess):

/Applications/Xcode.app/Contents/Developer/Library/Frameworks/Python3.framework/Versions/3.8/lib/python3.8/multiprocessing/popen_spawn_posix.py in __init__(self, process_obj)
     30     def __init__(self, process_obj):
     31         self._fds = []
---> 32         super().__init__(process_obj)
     33 
     34     def duplicate_for_child(self, fd):

/Applications/Xcode.app/Contents/Developer/Library/Frameworks/Python3.framework/Versions/3.8/lib/python3.8/multiprocessing/popen_fork.py in __init__(self, process_obj)
     17         self.returncode = None
     18         self.finalizer = None
---> 19         self._launch(process_obj)
     20 
     21     def duplicate_for_child(self, fd):

/Applications/Xcode.app/Contents/Developer/Library/Frameworks/Python3.framework/Versions/3.8/lib/python3.8/multiprocessing/popen_spawn_posix.py in _launch(self, process_obj)
     45         try:
     46             reduction.dump(prep_data, fp)
---> 47             reduction.dump(process_obj, fp)
     48         finally:
     49             set_spawning_popen(None)

/Applications/Xcode.app/Contents/Developer/Library/Frameworks/Python3.framework/Versions/3.8/lib/python3.8/multiprocessing/reduction.py in dump(obj, file, protocol)
     58 def dump(obj, file, protocol=None):
     59     '''Replacement for pickle.dump() using ForkingPickler.'''
---> 60     ForkingPickler(file, protocol).dump(obj)
     61 
     62 #

TypeError: cannot pickle 'weakref' object

Does anybody have an idea to remove this error?