Then hide it from the very beginning 
(and give it an internal name, like “__internal__”)
Whatever the reason is:
- It is poor UX (since several people got baffled by this bug)
- It should be easily fixed (I have been a lead web dev for like 18 years), e.g. hide it from the start and display a big “create deck” button
Anki seems to strongly excel in LTM training 
(I’m no expert in the domain, but it seems to be universally praised for that), and I’m aware that volunteers are doing a super great job with this free app, however I think it could be acceptable to say that there are a few UX issues in the webapp / desktop client that might benefit from just a bit of polishing 
(a few examples that comes to mind after a couple of hours of use: it makes no sense to separate the card-type “fields” and “cards” as they should be strongly linked, why limit to a single input field (for “type-in”), the menu is sometimes weird, when creating a custom study it should reset the filters when creating a new one (doesn’t seem to be the case, plus it should display how many tags are selected above the multiselect lists), things like that – and there is also the “default deck” thing, which should be called something like “hidden fallback deck”)
Known bugs can be documented too
Well in any case I wouldn’t want to hold forth on this, I just thought it could be improved, but if people are reluctant, and considering I am a total newcomer here, I will bow down and excuse myself (sincerely) gladly 
Thank you all for your input, have a great day!
Best,