Hi Marty,
I'd like to add some info, I hope it's relevant for your case:
See Note 1794460, for using the Variables input port on a BI Query. You should get no results for an invalid input value.
Regardless of the desired behavior on the BI level, it's possible to define a dropdown list for the input value, so the user can't go wrong (using an Entry List). I know it's not always relevant - If it's relevant for your case it might be a nice addition to the application.
Thanks,
Udi