Help needed with calculated query and count function

[AmyB]

I've created a totals query to count the number of places that have been booked on each course. In the same query, I would also like to display how many spaces are left on each course.

However, the expression: Places Left: Sum([Capacity]-[Booked Places]) gives me the error message "Can't specify subquery in expression ([Capacity]-[Booked Places]).

The SQL is as follows:
SELECT DISTINCTROW [SS Booking 2004/5].CourseID, SSAllCourses.CourseName, SSAllCourses.StartDate, Count([SS Booking 2004/5].[Student No]) AS [Booked Places], Sum([Capacity]-[Booked Places]) AS [Places Left]
FROM [SS Booking 2004/5] INNER JOIN SSAllCourses ON [SS Booking 2004/5].CourseID = SSAllCourses.CourseID
GROUP BY [SS Booking 2004/5].CourseID, SSAllCourses.CourseName, SSAllCourses.StartDate;

How can I resolve this? Any help is greatly appreciated.