INDEX and #REF error
 

Ok, I am using the INDEX function to sum a range that corresponds to a
changing column and row reference linked to another spreadsheet. It works
fine with the other spreadsheet open, but I get a #REF error when it is not.
Why won't it work with the sheet closed and is there a fix?

Heres the function as entered:
=(SUM(INDEX('O:\Prod\ScoreCard\[Score2004linked.xls]SCORE'!$B$11:AN$888,$B46,$G$1):INDEX('O:\Prod\Scor eCard\[Score2004linked.xls]SCORE'!$B$11:AN$888,$B46,$I$1)))

Thanks,
Neil