if #N/A then sum

[Jaakkolo]

I have two rows A1-A5 and B1-B5.
The A1-A5 gets values from another sheet and the B1-B5 copies them from the A1-A5.
My problem is that when for example A1 gets value #N/A I want the B1 to make a simple sum function C1+C2.
How do I do this?
I tried this to the B1:
=If(A1=#N/A;(C1+C2);A1)
I think this should work but #N/A probably has to be written in some other way?

Thanks for helping!

[Andy Pope]

=If(ISNA(A1);(C1+C2);A1)