That's a strange error, there shouldn't be any reason for it to show up. Is this on a new save file in v0.9.1? Are you running the game on a modern Windows PC? I've learned people run in to all sorts of strange bugs and glitches when they try to emulate Windows, so if you're not running a modern Windows machine that might be the issue.
Either way, I'll add more save slots in the future, as well as a more advanced debugger that will at least allow you to more easily regain lost progress.