You are trying to add a non-nullable field to category without a default

Возникла ошибка с миграциями в базе данных в django:

You are trying to add a non-nullable field 'url' to category without a default; we can't do that (the database needs something to populate existing rows).
Please select a fix:
 1) Provide a one-off default now (will be set on all existing rows with a null value for this column)
 2) Quit, and let me add a default in models.py
Select an option: fix
Please select a valid option: