OFFSET function and closed workbooks

[Berry Simpson]

I am building a workbook to pull rolling monthly averages
from five other workbooks. My formula uses the OFFSET
function to get the daily data from the other workbooks,
then uses AVERAGE to calculate the monthly average.
The OFFSET function works great if the other workbooks are
open, but crashes (shows #VALUE)when the other workbook
are closed.
Is there a problem with OFFSET pulling data from a closed
workbook?