diff --git a/classes/diskcache.php b/classes/diskcache.php
index 96c826728..2a3f8c8d7 100644
--- a/classes/diskcache.php
+++ b/classes/diskcache.php
@@ -3,6 +3,7 @@ class DiskCache implements Cache_Adapter {
 	/** @var Cache_Adapter $adapter */
 	private $adapter;
 
+	/** @var array<string, DiskCache> $instances */
 	private static $instances = [];
 
 	/**