sum函数内case语句中的SQL语法错误

下面是我的代码：

SELECT HOUR(asl.Start_Time) AS [Hour],
       t.team_name,
       SUM( CASE WHEN asl.Working = 1 
                 THEN asl.Duration_Seconds 
                 ELSE 0 END) 
           AS TotalSeconds
FROM (Agents 
         INNER JOIN Teams AS t 
         ON Agents.team_no = Teams.team_no)
      INNER JOIN AgentStateLogs AS asl 
      ON Agents.agent_no = asl.Agent_No
GROUP BY t.team_name, HOUR(asl.Start_Time);

问题是，它说的是语句中缺少一个运算符：

SUM( CASE WHEN asl.Working = 1 
          THEN asl.Duration_Seconds 
          ELSE 0 END) 
    AS TotalSeconds

我一直在努力寻找，但我看不出错误在哪里。有人能帮我解释一下吗？此代码在MS Access 2013中完成

先谢谢你

CASE WHEN asl.Working = 1 
THEN asl.Duration_Seconds 
ELSE 0 END

翻译成JET，这就变成了

IIf(asl.Working = 1, asl.Duration_Seconds, 0)

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@ta:d'oh。。。。羞愧地垂着头。。。