'Force single' Breaks Select List Filters

Using the core profile module, I create a 'list selection field' with several options - let's say, the numbers 1 to 12 (= months of the year).

In views, I create and expose a filter for this field. Options are:

* "is one of", and all 12 options ticked
* "optional" ticked (so we can see all users, if desired)
* "force single" ticked (so the user gets a dropdown menu, to select only one option)

With these options selected, the view funtions *only* if the user selects 'Any' from the dropdown menu. If the user actually selects one of the menu item (ie. one of the numbers from 1 to 12), then the filter does not work - ie. no users are ever listed, even though several users have been assigned to 'months'.

If I change the filter, unticking 'force single', now the user gets a selection list instead of a dropdown menu, and the query *does* work.

So, it looks like "force single" is making the view produce a bad query.

Can anyone confirm? - or am I missing something really obvious?