Expected a4 and a5 to be boxless.
Adding the box back at a4, and the WBS works. It does not matter if a5 uses a box or not.
The following generates the "Houston we have a problem" error.
(Note: Not a critical error for me, as I used an alternative. I just didn't want to let it slip past).
@startwbs
* a
** a2
*** a3
****_ a4
*****_ a5
@endwbs