diff --git a/lib/docomoru/client.rb b/lib/docomoru/client.rb
index <HASH>..<HASH> 100644
--- a/lib/docomoru/client.rb
+++ b/lib/docomoru/client.rb
@@ -19,7 +19,7 @@ module Docomoru
     include DialogueMethods
 
     # @param [String] api_key APIKEY issued from DoCoMo.
-    def initialize(api_key:)
+    def initialize(api_key: nil)
       @api_key = api_key
     end
Make it compatible with Ruby <I>