diff --git a/src/Client.php b/src/Client.php
index dbd4840..4714ce0 100644
--- a/src/Client.php
+++ b/src/Client.php
@@ -44,7 +44,7 @@ class Client
      */
     public function addHeader(string $key, string $value): self
     {
-        $this->headers[$key] = $value;
+        $this->headers[strtolower($key)] = $value;
         return $this;
     }