Null Value for RadioButton on Datawindow

[tas02]

I've got a field on a datawindow rendered as a RadioButton. I defined 3 values:

Servicable Null!
Remote R
Safety Issue S

I figured out that if I used "Null!" with the exclamation point, that clicking the Servicable button will indeed set the underlying field to null. However, PB doesn't show the button as selected (i.e. a dot in the circle).

Anybody have any suggestions on this. By the way, I'm using PB5 (I know, I know, don't say it!).

TIA...Tom