diff --git a/recaptcha-php/src/ReCaptcha/ReCaptcha.php b/recaptcha-php/src/ReCaptcha/ReCaptcha.php
index e2f7c34..b31494f 100644
--- a/recaptcha-php/src/ReCaptcha/ReCaptcha.php
+++ b/recaptcha-php/src/ReCaptcha/ReCaptcha.php
@@ -54,6 +54,10 @@ class ReCaptcha
      *
      * @param string $secret shared secret between site and reCAPTCHA server.
      * @param RequestMethod $requestMethod method used to send the request. Defaults to POST.
+     *
+     * @throws \RuntimeException
+     *   if no secret provided or if secret is not a string.
+     *
      */
     public function __construct($secret, RequestMethod $requestMethod = null)
     {