When running the `construct*.py` scripts, if there is no input data (e.g. when the ingest has failed), the error one gets tends to be a division by zero from the zero-length of the refList. This is more confusing to newbies and people unfamiliar with this part of the stack that one might expect, and is worth fixing I think. A simple check that one isn't operating on no data before going ahead should suffice I think.