diff --git a/src/Util/StringUtil.php b/src/Util/StringUtil.php
index 884bc40c..9b8325cf 100644
--- a/src/Util/StringUtil.php
+++ b/src/Util/StringUtil.php
@@ -28,7 +28,7 @@ public static function generateRandomBytes($size)
         } elseif (function_exists('openssl_random_pseudo_bytes')) {
             return openssl_random_pseudo_bytes($size);
         } elseif (function_exists('mcrypt_create_iv')) {
-            return mcrypt_create_iv($length);
+            return mcrypt_create_iv($size);
         }
         throw new \RuntimeException('Unable to create random bytes.');
     }