I am wondering why is not mentioned or not obvious in the documentation that the ref has to be numeric and the non-numeric characters are not accepted. it took me a while to find out!!
document ID portion of a reference is described, on this page, like so:
A document identifier, which is a string-encoded 64-bit integer.
But that description doesn’t appear everywhere that it should. Apologies for that.
I’ve filed an issue (DOCS-1850) to get that fixed. Hopefully, we’ll be able to address that soon.