[Python] Why does this async generator/anext snippet result in SystemError: <class...

Given this simple snippet, why does the first anext() call work but the second anext() results in a strange error?

Reading the documentation of anext(), I would assume that the supplied default would be used when there is no "next" value available:

If default is given, it is returned if the iterator is exhausted, otherwise StopAsyncIteration is raised.
​

Running this in an (interactive) Python 3.12 interpreter on MacOS Apple M3 Pro,

import asyncio

async def generator(it=None):
if it is not None:
yield (it, it)

async def my_func():
# results in a=1 b=1
a, b = await anext(generator(1), (2, 3))

# results in no printing
async for a, b in generator():
print(a, b)

# raises exception
a, b = await anext(generator(), (2, 3))

loop = asyncio.new_event_loop()
loop.run_until_complete(my_func())


results in this exception:

StopAsyncIteration

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "/Users/manuel/.pyenv/versions/3.12.0/lib/python3.12/asyncio/base_events.py", line 664, in run_until_complete
return future.result()
^^^^^^^^^^^^^^^
File "<stdin>", line 6, in my_func
SystemError: <class 'StopIteration'> returned a result with an exception set


My snippet fails in Python 3.11 & 3.13 too with the same exception message.

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