herokuでcakephpのキャッシュ設定したけどエラー出る

現状

herokuにて、下記サイトを参考にキャッシュ設定を行なったが、ログを見るとエラーが吐かれてます
https://qiita.com/tektoh/items/90f533c7f2a66e4a3fad

このエラーを消す方法がわかりません。

環境

cakephp 3.6
heroku

add-on : memcachier

エラー

// 該当エラーログ(composer.jsonにてモジュール追加前)
2018-11-25 16:39:08 Warning: Warning (512): Cache engine Cake\Cache\Engine\MemcachedEngine is not properly configured. in [/app/vendor/cakephp/cakephp/src/Cache/Cache.php, line 176]

// 該当エラーログ(composer.jsonにてモジュール追加後)
2018-11-25T08:44:54.442017+00:00 app[web.1]: 10.43.205.53 - - [25/Nov/2018:17:44:53 +0900] "GET / HTTP/1.1" 500 33 "https://dashboard.heroku.com/" "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/70.0.3538.102 Safari/537.36

ソース

// composer.json
   "require": {
        "php": ">=5.6",
        "cakephp/cakephp": "3.6.*",
        "cakephp/migrations": "^2.0.0",
        "cakephp/plugin-installer": "^1.0",
        "josegonzalez/dotenv": "3.*",
        "mobiledetect/mobiledetectlib": "2.*",
        "ext-memcached": "*" // モジュールを追加
    },
    "require-dev": {
        "cakephp/bake": "^1.1",
        "cakephp/cakephp-codesniffer": "^3.0",
        "cakephp/debug_kit": "^3.15.0",
        "psy/psysh": "@stable",
        "heroku/heroku-buildpack-php": "*"
    },

// config/app_heroku.php app.phpの上書きファイル
/**
 * Configure the cache adapters
*/
     'Cache' => [
         'default' => [
             'className' => 'Cake\Cache\Engine\MemcachedEngine',
             'prefix' => 'myapp_cake_',
             'servers' => [env('MEMCACHIER_SERVERS')],
             'username' => env('MEMCACHIER_USERNAME'),
             'password' => env('MEMCACHIER_PASSWORD'),
             'duration' => '+1440 minutes',
         ],

         'session' => [
             'className' => 'Cake\Cache\Engine\MemcachedEngine',
             'prefix' => 'myapp_cake_session_',
             'servers' => [env('MEMCACHIER_SERVERS')],
             'username' => env('MEMCACHIER_USERNAME'),
             'password' => env('MEMCACHIER_PASSWORD'),
             'duration' => '+1440 minutes',
         ],

         '_cake_core_' => [
             'className' => 'Cake\Cache\Engine\MemcachedEngine',
             'prefix' => 'myapp_cake_core_',
             'servers' => [env('MEMCACHIER_SERVERS')],
             'username' => env('MEMCACHIER_USERNAME'),
             'password' => env('MEMCACHIER_PASSWORD'),
             'duration' => '+1 years',
         ],

         '_cake_model_' => [
             'className' => 'Cake\Cache\Engine\MemcachedEngine',
             'prefix' => 'myapp_cake_model_',
             'servers' => [env('MEMCACHIER_SERVERS')],
             'username' => env('MEMCACHIER_USERNAME'),
             'password' => env('MEMCACHIER_PASSWORD'),
             'duration' => '+1 years',
         ],
     ],