diff --git a/lib/classes/JsonApi/Routes/Files/AbstractRangeIndex.php b/lib/classes/JsonApi/Routes/Files/AbstractRangeIndex.php
index ec4546ecaf3bbc897a2d9980215f29bc84b5bfee..15a98c3b414ce72e648ecd9e595a05a7b26ac6b2 100644
--- a/lib/classes/JsonApi/Routes/Files/AbstractRangeIndex.php
+++ b/lib/classes/JsonApi/Routes/Files/AbstractRangeIndex.php
@@ -73,7 +73,8 @@ abstract class AbstractRangeIndex extends JsonApiController
             if ($topFolder->isVisible($userId)) {
                 $folders[$topFolder->getId()] = $topFolder;
                 if ($topFolder->isReadable($userId)) {
-                    array_walk($topFolder->getSubFolders(), $arrayWalker);
+                    $subfolders = $topFolder->getSubFolders();
+                    array_walk($subfolders, $arrayWalker);
                 }
             }
         };