sum function returns "N/A" value!!

Hi

I'm trying to use the sum function on an array.
The problem is, some values in this array are "N/A" (the result of a VLOOKUP). I'm happy to live with these N/A values, but when I try to sum up the array, I get an "N/A" as well!! Excel's Help says that the sum funtion will not sum up error values or text if they've been TYPED in; i guess i have the problem cos the error values were the result of another funtion. Is there any way I can solve this problem?

Thanx!!