diff --git a/lib/private/Files/Cache/Scanner.php b/lib/private/Files/Cache/Scanner.php
index 8ff4d1ab19b6..be3847625930 100644
--- a/lib/private/Files/Cache/Scanner.php
+++ b/lib/private/Files/Cache/Scanner.php
@@ -431,7 +431,7 @@ private function handleChildren($path, $recursive, $reuse, $folderId, $lock, &$s
 					} elseif ($data['mimetype'] === 'httpd/unix-directory' and $recursive === self::SCAN_RECURSIVE_INCOMPLETE and $data['size'] === -1) {
 						// only recurse into folders which aren't fully scanned
 						$childQueue[$child] = $data['fileid'];
-					} elseif ($data['size'] === -1) {
+					} elseif (!isset($data['size']) || $data['size'] === -1) {
 						$size = -1;
 					} elseif ($size !== -1) {
 						$size += $data['size'];