Attached a patch. Rather than altering choices or making a special check for string instances, I just changed the if statement to

if action.choices is not None and value not in list(action.choices):

from 

if action.choices is not None and value not in action.choices:

It has the added benefit of handling all sequence types correctly (rather than just strings). I tried to think of a case where this wouldn't work as expected, but wasn't able to.