The Response content must be a string or object implementing __toString() using Model::all()

I have two simple endpoints that are basically returning all rows from the controller like

public function index()
{
    return Benefit::all();
}

As you can see, I'm letting laravel magic handle the conversion to json, and while one of the endpoints works, this one does not, giving error

UnexpectedValueException: The Response content must be a string or object implementing __toString(), "boolean" given.

I'm assuming it has something to do with not being able to convert some of the values to string ? However, all fields in the table are varchar.

How can I debug which row(s) are causing this ?

thanks