Check the primary key of your users database. If you set your table's primary key other than id, you need to set it in your User model.

The Laravel Documentation states the following:

Eloquent will also assume that each table has a primary key column named id. You may define a protected $primaryKey property to override this convention.

For example, if you have a user_id column set as your primary key in your users database, you need to place the code below in the User model:

protected $primaryKey = 'user_id';