Source: WowAce

In all likelihood, invalid data is still stored in the addon's database and must be manually purged. Since users may not be able to do this easily, a simple cleanup / check could be added to avoid the errors and make sure only valid attempts data will be saved in the future.

I instead opted for preventing the error messages and not messing with the user's DB.

While not ideal, this is much safer and shouldn't cause any adverse effects by allowing the "wrong" attempt count to be displayed until it's updated next.