Thanks for the report! This is a cloud bug indeed. Incidentally, we're already working on an overhaul of our client-side input tokenizer (where this is currently going wrong) that will fix this issue. I've still filed a bug ticket in our system (just for reference: CLOUD-15982).
As a workaround for now, you could use an explicit Optional wrapper, i.e.
Optional[a_]
instead of:
a_.
