diff --git a/phpBB/phpbb/db/migration/data/v400/rename_duplicated_index_names.php b/phpBB/phpbb/db/migration/data/v400/rename_duplicated_index_names.php
index 805d68ff75..f2d8107a9b 100644
--- a/phpBB/phpbb/db/migration/data/v400/rename_duplicated_index_names.php
+++ b/phpBB/phpbb/db/migration/data/v400/rename_duplicated_index_names.php
@@ -27,7 +27,7 @@ class rename_duplicated_index_names extends migration
 	/**
 	 * @var array
 	 */
-	protected static $rename_index;
+	protected static $rename_index = [];
 
 	public static function depends_on()
 	{