This means that H9 is non-empty and contains a non-numeric value. So I am
guessing that H9 evaluates to "" (an empty string).
You could try this in H9:
=IF(D3=0,0,D3+179)
But this will show a 0 instead of ""
If showing a 0 is not possible, you could use this in H13:
=if(isnumber(h9),h9+1,"")
--
Dave Peterson