For some reason FB treats a "Choose One" with only one option as an error ("Choices : You must have at least one choice").
Maybe it's an off by one and that error is meant to trigger when there are no options at all?. In my case I'm dynamically constructing the list of choices, and sometimes it turns out that there's only one choice, which works fine at runtime. Then the next time I can't run the build because there's only 1 option...
(IMHO the build should still run even if there are no choices, and it should just assign a blank value to the variable)
Steve