Adding cells

I have a form that has 10 cells that take a number input. The problem I am having is that not all of the cells will always have a number entered by the user. In fact most of the time it will be around 2-3. I assume what is going wrong is that the formula is trying to add void cells since there is not data inputed into them by the user. I tried making the initial value 0, but apparently that does not roll over either. Does anyone know how to fix this problem?