Hi nuoem ,
Sorry for any inconvenience caused.
That is very weird. I've just tried to reproduce it, and it worked for me.
Please, activate your PHP error log, set the stock to a positive value, and click on the Save button. Then set again the value to 0 and save it again. When done, please take a look at your PHP error log. If any errors are logged, paste the text here.
If you don't know how to activate your error log, please, read this article.
Best Regards,
José Andrés