Required multi-select list box

I want to create a required multi-select list box, but it doesn't work.  I am not doing anything tricky, just using the LiveCycle Designer ES UI (8.2.1.4029.1.523496).  I've triple-checked my work and can't figure out why it isn't working (type = list box; allow multiple selection is checked; type = user entered - required; default = < None >).

It displays with a red box around it to indicate that it is required, there is no default value selected as expected, yet I can submit the form without the required validation firing.  All other required fields work as expected.

Is there some trick I'm missing?

Thanks,

David