YEAR, MONTH, DAY strul

Vad är problemet med den här SQL:en?

SELECT YEAR(a1.ActivityStartDate) AS ActivityStartYear
, MONTH(a1.ActivityStartDate) AS ActivityStartMonth, DAY(a1.ActivityStartDate) AS ActivityStartDay
[blue], (SELECT COUNT(*) FROM tblM_Activity a2 WHERE YEAR(a2.ActivityStartDate) = YEAR(a1.ActivityStartDate)
 AND MONTH(a2.ActivityStartDate) = MONTH(a1.ActivityStartDate) AND DAY(a2.ActivityStartDate) = DAY(a1.ActivityStartDate)) -- raden med felet[/blue]
 FROM tblM_Activity a1
 WHERE YEAR(a1.ActivityStartDate) = 2002 AND MONTH(a1.ActivityStartDate) = 09
 GROUP BY YEAR(a1.ActivityStartDate), MONTH(a1.ActivityStartDate), DAY(a1.ActivityStartDate)
 ORDER BY YEAR(a1.ActivityStartDate), MONTH(a1.ActivityStartDate), DAY(a1.ActivityStartDate)

Jag får felet:

Server: Msg 8630, Level 17, State 38, Line 1
Internal Query Processor Error: The query processor encountered an unexpected error during execution.