Laravel Excel 3.1 throws a "property doesnt have a default value" error despite importing successfully

I am using the Laravel Excel package to handle bulk uploads. Whilst Im able to get the data to upload successfully, my web console indicates and error that 'staff_id' doesn't have a default value. I have tried to catch this as an exception but this does not get triggered. I am using the ToModel import as indicated below

class EmployeesImport implements ToModel, WithHeadingRow
{
    public function model(array $row)
    {
        try {
            return new Employee([
                'staff_id' => $row['staff_id'],
                'first_name' => $row['first_name'],
                'middle_name' => $row['middle_name'],
                'last_name' => $row['last_name'],
                'national_id' => (string) $row['national_id'],
                'department_id' => 1,
            ]);
        } catch (\Exception $e) {
            dd($e->getMessage(), $row);
        }
    }
}

The CSV Im importing has the following structure

Within my controller, I have this to exceute the upload/import

Excel::import(new EmployeesImport(), request()->file('bulk'));

And finally, this is my Employees Model, showing the fillable fields

class Employee extends Model
{
    use SoftDeletes;

    protected $table = "employees";

    protected $fillable = [
        "staff_id", "first_name", "middle_name", "last_name", "national_id", "department_id", "avatar"
    ];
}

(One last thing) In case it may hold relevance - my migration file's up method

public function up()
{
    Schema::create('employees', function (Blueprint $table) {
        $table->bigIncrements('id');
        $table->string('staff_id')->unique();
        $table->string('first_name');
        $table->string('middle_name')->nullable();
        $table->string('last_name');
        $table->string('national_id')->unique();
        $table->unsignedBigInteger('department_id');
        $table->longText('avatar')->nullable();
        $table->timestamps();
        $table->softDeletes();

        //Foreign keys
        $table->foreign('department_id')->references('id')->on('departments')->onDelete('cascade');
    });
}

via Chebli Mohamed