diff --git a/src/ConfigInstaller.php b/src/ConfigInstaller.php
index 0481f6507afc6de75d767d21a6b2e56566593ccb..b15a68e68602b9c9359af02a91c02fe028cb77be 100644
--- a/src/ConfigInstaller.php
+++ b/src/ConfigInstaller.php
@@ -138,7 +138,8 @@ class ConfigInstaller extends OriginalConfigInstaller {
         $this->updater->doExecuteUpdate($data["items"]);
       }
 
-      $alterations_applied->set("files", $alterations_applied->get("files") + array_keys($config_to_alter))->save();
+      $applied_files = array_merge($alterations_applied->get("files"), array_keys($config_to_alter));
+      $alterations_applied->set("files", $applied_files)->save();
     }
   }