diff --git a/lib/apple_id/client.rb b/lib/apple_id/client.rb
index c1907b3..ba060fc 100644
--- a/lib/apple_id/client.rb
+++ b/lib/apple_id/client.rb
@@ -34,7 +34,7 @@ def client_secret_jwt
         exp: 1.minutes.from_now
       )
       jwt.kid = key_id
-      jwt.sign(private_key)
+      jwt.sign(private_key).to_s
     end
 
     def setup_required_scope(scopes)