Не могу выполнить функцию Update

UPDATE Customer
SET Customer.NumberOfActiveSubs = (SELECT
        SUM(CASE WHEN Subs.SubEndDate >= CONVERT(date, GETDATE()) THEN 1 ELSE 0
        END) as ActiveSubs
    FROM Subscription as Subs
    INNER JOIN Customer as Cust
        ON Subs.CustomerID = Cust.CustomerID
    WHERE Cust.CustomerID = Subs.CustomerID
    Group by Cust.CustomerID)

Ошибка:

Subquery returned more than 1 value. This is not permitted when the subquery follows =, !=, <, <= , >, >= or when the subquery is used as an expression. The statement has been terminated.