diff --git a/src/Api.php b/src/Api.php
index 9f85753..1fc42e7 100644
--- a/src/Api.php
+++ b/src/Api.php
@@ -32,7 +32,7 @@ public function __construct(PinDuoDuo $pinduoduo, $needToken = false)
      *
      * @return string
      */
-    private function signature($params)
+    private function signature($params): string
     {
         ksort($params);
         $paramsStr = '';