diff --git a/src/S3/StreamWrapper.php b/src/S3/StreamWrapper.php
index 241370e956..333956d073 100644
--- a/src/S3/StreamWrapper.php
+++ b/src/S3/StreamWrapper.php
@@ -515,6 +515,8 @@ private function formatKey($key)
      */
     public function rename($path_from, $path_to)
     {
+        // PHP will not allow rename across wrapper types, so we can safely
+        // assume $path_from and $path_to have the same protocol
         $this->initProtocol($path_from);
         $partsFrom = $this->withPath($path_from);
         $partsTo = $this->withPath($path_to);