diff --git a/src/RetryMiddleware.php b/src/RetryMiddleware.php
index 75a8574aa..5acc8c5c3 100644
--- a/src/RetryMiddleware.php
+++ b/src/RetryMiddleware.php
@@ -47,7 +47,7 @@ public function __construct(
      *
      * @param int $retries
      *
-     * @return int
+     * @return int milliseconds.
      */
     public static function exponentialDelay($retries)
     {