diff --git a/phpBB/includes/cache/driver/memory.php b/phpBB/includes/cache/driver/memory.php
index 92971c6cb2..e0771ab1d3 100644
--- a/phpBB/includes/cache/driver/memory.php
+++ b/phpBB/includes/cache/driver/memory.php
@@ -19,7 +19,7 @@ if (!defined('IN_PHPBB'))
 * ACM Abstract Memory Class
 * @package acm
 */
-class phpbb_cache_driver_memory extends phpbb_cache_driver_base
+abstract class phpbb_cache_driver_memory extends phpbb_cache_driver_base
 {
 	var $key_prefix;