Got it wrong for the best possible reason: I learned something.
Like many others, I thought a FULL backup was required to re-initiate the log chain, probably because that's what most recommendations say, following a switch of Full-to-Simple-to-Full recovery modes.
Here's an example from a SQL MVP (http://www.petri.co.il/sql-server-recovery-models.htm):
...if you’re in full mode and you want to truncate the log by switching to simple mode you’re free to do that. However, if you want to switch back to full mode and start taking log backups again, you’ll have to take a full backup before you can do that.
Great question, Steve!
Rich