=CELL("filename")
You should add a cell reference to this function.
=CELL("filename",A1)
Otherwise, the result will be the active workbook when a
calculation is made, which may not be the same workbook that
contains the formula. Including a cell reference -- any cell will
do -- will prevent this error.
--
Cordially,
Chip Pearson
Microsoft MVP - Excel
Pearson Software Consulting, LLC
www.cpearson.com