When you create a list field bind to user groups, a database table store bind ids between the field and user groups. These ids begin from 1, but id 100 have a special meaning (None value). If a bind id use it, when you set None value it will understand another value (a user group).

This problem doesn't exist for static list because bind ids begin from 101.

• Old platforms are tricky to fix, because we cannot know if a field set to 100 means None or a user group.
• New platforms can be fixed by setting initial id to 101 as for static list fields.