How to display simply zero if the the divisor of a calculated field is zero?

Good day!

I am doing a fillable PDF. In this document, I have a Reset Button with the following script ...

if(4==app.alert("This action will clear all input fields in this form.\nDo you want to continue?",1,2))this.resetForm()

In this document, I have a calculated formula  as showned below:

(FI017a+FI018a)/(FI017a+FI017b+FI018a+FI018b)*100

When the Reset Button is click, all the numeric fields will be set to zero thus making the operants of the formula zero including the divisor.

I believe this cause the error ...

"The value entered does not match the format of the field [fieldname].

How can I show zero as the result of the computation and disregard the error message.

Appreciate the help. Thank you.