Does this work for

    help='choices: %(choices)s'

Without setting up my own test case, I'm guessing that the problem is with an empty help string.  `help=''` should be ok, but the test handle that probably occurs earlier.

Choices, empty or not, are included earlier in the help line, as well as usage.  So there isn't real need to include them in the user defined part of the help line.

Still, there are other bug issues regarding the formatting of choices.  That part needs refinement to handle very long lists of choices, as well as ones that don't fit the usual 'list' format.