Iserror does not work for missing sheets in linked file

Hello,

I have a file with links to another file. The formula I have tried
is:
IF(ISERROR(SUM('C:\Temp\mainfile.xls]sheet1'!$AT:$AT)),0,SUM('C:\Temp
\otherfile.xls]sheet1'!$AT:$AT))

If sheet1 does not exist in the otherfile.xls, I get one of two
events:
1. If otherfile.xls file is open I get the error message "Your formula
contains an invalid external reference to a worksheet."
2. If otherfile.xls is closed, I get the dropdown where I can chose
among the sheets in the otherfile.xls.

I want the function to return a zero in case of missing sheet. Is this
possible?


Thanks and regards,
Vegard

Update:
Wrong example formula, the right one is of course:
IF(ISERROR(SUM('C:\Temp\otherfile.xls]sheet1'!$AT:$AT)),0,SUM('C:\Temp
\otherfile.xls]sheet1'!$AT:$AT))