Excel GetCell "Type mismatch"

[TheVig]

Error message:
"Error (RT) in Balance_Check: 0x80020005, Type mismatch."

The data in the cell I am trying to retrieve a value from is derived from the following formula:
=IF(A4="","",SUM(Data!$C$2:C4)-SUM($D$24))

Function Balance_Check

Code:

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str sfile="$desktop$\ExcelFile.xls"
str asheet="Sheet1"
Excel.Application app._create
Excel.Workbook book=app.Workbooks.Open(_s.expandpath(sfile))
ExcelSheet es.ws=book.Worksheets.Item(asheet)

str Value
es.GetCell(Value 6 4)

It's driving me nuts! I can grab values that are static values, and most that have values derived from formulas. It seems like the "IF" statement really screws things up.

Thanks,
Jimmy Vig