diff --git a/src/Handler/GuzzleV5/GuzzleHandler.php b/src/Handler/GuzzleV5/GuzzleHandler.php
index 7ab87569d3..c75d5161b7 100644
--- a/src/Handler/GuzzleV5/GuzzleHandler.php
+++ b/src/Handler/GuzzleV5/GuzzleHandler.php
@@ -28,6 +28,7 @@ class GuzzleHandler
     private static $validOptions = [
         'proxy'             => true,
         'expect'            => true,
+        'cert'              => true,
         'verify'            => true,
         'timeout'           => true,
         'debug'             => true,