Laravel: How to check every element attribute value of an array of JSON data in unit testing

Question

I am trying to test if the filter response has the right JSON data (every item should have the filtered group_code). I tried the following code but it doesn't work.

 public function test_filter_items_by_group_code_work_properly()
 {
    $filterCode = '026';

    $response = $this->get($this->ITEMS_URL . '?' . $this->groupAttribute . '='. $filterCode);

    $response->assertStatus(200);

    $items = $response->json()['data'];
    
    foreach ($items as $item) {
        $item_group = strval($item['group_code']);
        self::assertJson($item_group, $filterCode);
    }
  }

I get the following error:

Illuminate\Foundation\Bootstrap\HandleExceptions::handleError("Trying to access array offset 
on value of type int",

The JSON response structure is like that:

{
"message": "Items fetched successfully.",
"data": {
    "current_page": 1,
    "data": [
        {
            "group_code": "026",
             ...
        },
        ...