I'm attempting to do a simple 3D sum of data from multiple worksheets within a workbook. Every time I attempt the command "=sum("sheets")" I get an error message: "Your formula contains an invalid external reference. Verify that the path, workbook, and range name or cell reference are correct, and try again." If I manually put in a formula to sum the same cells from the same sheets it works fine, but I'd prefer not to do the extra work. Any suggestions?