diff --git a/app/controllers/api/v1/users_controller.rb b/app/controllers/api/v1/users_controller.rb
index 26f24129a..9632fdfc1 100644
--- a/app/controllers/api/v1/users_controller.rb
+++ b/app/controllers/api/v1/users_controller.rb
@@ -5,7 +5,7 @@ class Api::V1::UsersController < Api::V1::BaseController
   before_action :check_auth_for_observe_user, except: [:check_user_id, :check_user_login, :warning_user_ids]
 
   def warning_user_ids 
-    warning_user_ids = EduSetting.get("warning_user_ids")
+    warning_user_ids = EduSetting.get("warning_user_ids") || ""
     render_ok({data: warning_user_ids.split(",")})
   end