Cannot use object of type Illuminate\Http\JsonResponse as array

I was working with PHPUnit to test my invoice, so before I implement testing to my app I upgrade my Laravel application from 6 to version 7. and I follow everything in the documentation of the upgrade. in last PHPUnit, it's working but all of my APIs return response like so

return response()->json([
                'status' => false,
                'message' => "Customer is not exist for this vendor {$vendor_id}"
            ], 404);

and when I run the test is saying

Tests\Feature\InvoiceTest::salesman_can_store_invoice
Error: Cannot use object of type Illuminate\Http\JsonResponse as an array

C:\xampp\htdocs\backend\app\Http\Controllers\api\v1\InvoiceApiController.php:321

unitTest

$response = $this->withHeaders([
            'AuthId' => $driver->id,
            'AuthToken' => $driver->authToken,
        ])->postJson('/api/store_invoice', [$params]);


$response->assertExactJson([
                'status' => false,
                'message' => "Customer is not exist for this vendor 10"
        ], 404);

and when I return a response as an array it's working. so I want to know why it's not working when my response looks like I mentioned above?