want to sumdivided instead of sumproduct that contains blanks.

Hi

As you can see in the attached file I have a problem where I have
A column that I need to be able to choose wheter the row is interesting or not.
Two columns that I need to sum up the divided product.

Had it been that I needed to sum the product I could have used a sumproduct. The same for summing up divided products unless as in my case now where I have blank cells, giving me #DIV/0!
Anyone know of any way to work around that problem?

Formula how it looks right now not working: =SUMPRODUCT(N(A:A="a");B:B;1/C:C)