diff --git a/api/src/Vfs.php b/api/src/Vfs.php
index 291407118a..931975dd37 100644
--- a/api/src/Vfs.php
+++ b/api/src/Vfs.php
@@ -1909,6 +1909,10 @@ class Vfs extends Vfs\Base
 	 */
 	static public function copy_files(array $src, $dst, &$errs=null, array &$copied=null)
 	{
+		if (!is_array($copied))
+		{
+			$copied = [];
+		}
 		if (self::is_dir($dst))
 		{
 			foreach ($src as $file)