diff --git a/lib/rucaptcha/controller_helpers.rb b/lib/rucaptcha/controller_helpers.rb
index fe187e3..4061f2a 100644
--- a/lib/rucaptcha/controller_helpers.rb
+++ b/lib/rucaptcha/controller_helpers.rb
@@ -9,6 +9,7 @@ module RuCaptcha
     # session key of rucaptcha
     def rucaptcha_sesion_key_key
       session_id = session.respond_to?(:id) ? session.id : session[:session_id]
+      warning_when_session_invalid if session_id.blank?
       ['rucaptcha-session', session_id].join(':')
     end
 
@@ -73,5 +74,12 @@ module RuCaptcha
       end
       false
     end
+
+    def warning_when_session_invalid
+      Rails.logger.warn "
+        WARNING! The session.id is blank, RuCaptcha can't work properly, please keep session available.
+        More details about this: https://github.com/huacnlee/rucaptcha/pull/66
+      "
+    end
   end
 end