Thank you Nathanel !!

You nailed it! 

I didn't want to be stuck with a list of new invoices creating the same issue, so I kept checking back to my last saved invoice where this problem with the message arose.  

After reading your answer it made sense and I courageously went ahead and made up a new invoice, saved, and Previewed - Voilà! -it was good.   

I hope all the other users have seen  your answer.