Saving with a co-pro enabled seems to crash jsbeeb/brings up the debugger

[ZornsLemma]

With the current code on xania.org, firing up a BBC B with 65C02 co-pro, leaving the default Elite disc in the drive and doing:

*SAVE Z 1000 1100

starts to save and then brings up what looks like the jsbeeb debugger:

Credit for discovering this goes to Fredrik, who came across it while testing some of my code on jsbeeb over on stardot. All errors in this bug report are my own, of course. :-)

This works fine if the co-pro is disabled.