Regrettably, this does look like a possible problem. The test writes out an unpopulated SegmentedFile.Builder, i.e. that has no boundaries, which should not really happen in a live system (and is required for any problem to occur). However it looks like we do write out such an unpopulated set of boundaries, during index summary downsampling.
Apologies for missing that. The fix is beyond trivial, I have ninja'd it in.
