diff --git a/index.js b/index.js
index d2061ef..d360022 100644
--- a/index.js
+++ b/index.js
@@ -67,7 +67,7 @@ function checkIsSecretKey(key) {
   }
 
   if (typeof key === 'string') {
-    return key;
+    return;
   }
 
   if (!supportsKeyObjects) {