In that case, it would probably be easier to eliminate the #N/A errors rather
than trying to SUM around them.
Do something like:
=IF(ISNA(your formula),"",your formula)
This way, if your formula results in #N/A, a blank will be returned instead.
Then, the SUM function will work on those cells. SUM(A1,A5,A12)
HTH,
Elkar