Help!!

I am trying to sum cells in different worksheets to get a total on my
summary page but it will not add the cells.

This is the formula:
=SUM('Jan-Feb 09'!C8, 'Mar-Apr 09'!C8, 'May-June 09'!C8, 'July-Aug 09'!C8,
'Sept-Oct 09'!C8, ' Nov-Dec 09'!C8)

Why won't it calculate the cells?
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cjudge