I found what appears to be a very similar issue so instead of creating a new issue I will place it here as a comment.

The following code:

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import argparse

parser = argparse.ArgumentParser()
parser.add_argument('letter', choices=['a', 'b', 'c'], default=argparse.SUPPRESS, nargs='?')
args = parser.parse_args([])
==

results in this error:

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usage: pok.py [-h] [{a,b,c}]
pok.py: error: argument letter: invalid choice: '==SUPPRESS==' (choose from 'a', 'b', 'c')
==