Xpage: Partial refresh error - can't find the element

I have 2 radio buttons in a group with a computed field beneath them that displays different help text based on the radio button selection. So, each radio button is set to partial refresh the computed field element. However, when I click one of the radio buttons, I get an error:

No element to submit with id view:_id1:_id26:DescTab:computedField2

At first, I just had a label control with a computed value, but then tried the computed field instead. Both give me the same result.

What am I missing?

tia

*update - I pulled the radio buttons off my custom control and put them in a blank xpage. It works as expected there, so it must have something to do with the fact that my design has the radio buttons in a section in a tabbed table in a custom control on an xpage.