diff --git a/src/Lunr/Corona/Exceptions/NotImplementedException.php b/src/Lunr/Corona/Exceptions/NotImplementedException.php
index a778e4cc..8a8bc5fc 100644
--- a/src/Lunr/Corona/Exceptions/NotImplementedException.php
+++ b/src/Lunr/Corona/Exceptions/NotImplementedException.php
@@ -22,11 +22,11 @@ class NotImplementedException extends HttpException
     /**
      * Constructor.
      *
-     * @param string|null    $message  Error message
+     * @param string         $message  Error message
      * @param int            $app_code Application error code
      * @param Exception|null $previous The previously thrown exception
      */
-    public function __construct(?string $message = NULL, int $app_code = 0, Exception $previous = NULL)
+    public function __construct(string $message = 'Not implemented!', int $app_code = 0, Exception $previous = NULL)
     {
         parent::__construct($message, HttpCode::NOT_IMPLEMENTED, $app_code, $previous);
     }