diff --git a/src/Enum.php b/src/Enum.php
index ce0dd80..deac0ed 100755
--- a/src/Enum.php
+++ b/src/Enum.php
@@ -119,7 +119,8 @@ public static function isValid($value)
      */
     public static function isValidKey($key)
     {
-        return in_array($key, self::keys(), true);
+        $array = self::toArray();
+        return isset($array[$key]);
     }
 
     /**