I think the best thing would be to have IFERROR functions around the formulas in those cells
e.g if the formula in D3 =C2/D2, you can use IFERROR(C2/D2,0) or IFERROR(C2/D2,"") that way, an error would default to zero, so that your sum is unaffected
cos to start writing IFERROR(D3,0)+IFERROR(F3,0)+IFERROR(I3,0) doesn't look so cool, but would work