php - array_column がオブジェクトの配列で機能しない?

Question

これは私の現在の配列構造です:

array(2) {
  ["Ground"] => array(4) {
    [0] => object(Shipping\Model\Shipping)#337 (5) {
      ["shipping_id"] => NULL
      ["min_weight"] => string(4) "0.00"
      ["max_weight"] => string(4) "5.00"
      ["shipping_method"] => string(6) "Ground"
      ["shipping_rate"] => string(4) "8.00"
    }
    [1] => object(Shipping\Model\Shipping)#385 (5) {
      ["shipping_id"] => NULL
      ["min_weight"] => string(4) "6.00"
      ["max_weight"] => string(5) "10.00"
      ["shipping_method"] => string(6) "Ground"
      ["shipping_rate"] => string(5) "12.00"
    }
  }
  ["Expedited"] => array(4) {
    [0] => object(Shipping\Model\Shipping)#388 (5) {
      ["shipping_id"] => NULL
      ["min_weight"] => string(4) "0.00"
      ["max_weight"] => string(4) "5.00"
      ["shipping_method"] => string(9) "Expedited"
      ["shipping_rate"] => string(5) "12.00"
    }
  }
}

array_columnforを実行するたびにmax_weight、結果として空の配列が得られます。に返された配列を使用することになっていますmax()。これに対する回避策はありますか?