diff --git a/src/DataLoaderInterface.php b/src/DataLoaderInterface.php
index 0a2766e..0cad76b 100644
--- a/src/DataLoaderInterface.php
+++ b/src/DataLoaderInterface.php
@@ -16,7 +16,7 @@ interface DataLoaderInterface
     /**
      * Loads a key, returning a `Promise` for the value represented by that key.
      *
-     * @param string $key
+     * @param mixed $key
      *
      * @return mixed return a Promise
      */