Why doesn't firstOrCreate work in laravel?

Question

Laravel

I have my own table of cities and regions.
When registering, the user can select a city from the list or add a new one.
In the controller, I check if there is a variable with the new city and, if so, I call:

$city = City::firstOrCreate([
  'name' => $request->new_city,
  'region_id' => $request->region_id
]);

If there is a city with the same name and region id, then everything is fine, but if not (only region_id matches), then the answer comes with an error:

What could be the problem? There are no specifics (at least, understandable to me) in the answer. Just the handler complains about the name field

Answer 1

https://laravel.com/docs/5.6/eloquent#mass-assignment