Ошибка в коде SQL

SELECT COUNT(DISTINCT Subscriber_idSubscriber)
FROM (
  SELECT idPlan, Communication_idCommunication
  FROM Plan
  WHERE Communication_idCommunication = 1
) AS A
INNER JOIN Subscriptions AS B ON A.Plan_idPlan = B.Plan_idPlan;

Что значит? Error Code: 1054. Unknown column 'A.Plan_idPlan' in 'on clause' 0.000 sec Как это исправить?