Your code runs fine under 3.6.5.

But if I add 'subparsers.required=True', I get your error.  It's having problems formatting the name of the subparsers command when issuing the error message.

If I add a 'dest' to the add_subparsers I get the expected error message:

    import argparse

    parser = argparse.ArgumentParser(description="Automatically process XML")
    subparsers = parser.add_subparsers(title='subcommands', dest='cmd')
    subparsers.required=True
    chain_parser = subparsers.add_parser('chain', aliases=['c'], 
        help='Automatically run a chain of transformations')

    args = parser.parse_args()
    print(args)

The default 'dest' for subparsers is None, and the error arises from

    ','.join([None])

The problems with a missing 'dest' came up in earlier discussions about making required/not required subparsers.  3.7 made subparsers 'required' by default, but it appears that it hasn't addressed this missing 'dest' problem.

But I haven't followed the latest release details closely.