ALAN and others,
Please don't post the same question in multiple places. You just waste the time of people who are trying to help YOU. There is another thread for this question where it was answered.
To recap, the problem was caused by a divide by 0 exception so you need to prevent the division operation when the divisor is 0 or null.
IIf(Nz(YourField1,0) = 0, 0, YourField2/YourField1)