Currently it check if value is numeric or not. You can set default value to 0 (zero).
Next release i will fix it so that user can post empty value also.
I have tested with my test server and it is working here. That means something in your website preventing it to function.
Can you create a test user account in your website for me so that i can access your website and test there?
If yes, send credentials using https://www.shamimsplugins.com/sensitive-information/
Please check and let me know following
1. Did you click “Apply” after selecting delete?
2. Can you disable all other plugins and change theme to one of default theme and try? Then enable one by one.