In all the versions of MS Execel I have seen, the result is "#NA" and if that value is used n another calculation it is omitted from the calculation but included in the count. Since Acrobat and LiveCycle form fields with a format of "Number" do not accept "NaN", "Infinity", "-Infinity" or scientific notation for very small or very large numbers. The values for Infinity and scientific notation both have a JavaScript type of "number" and not "string" so they could be used in a calculation with some work.
A possible custom calculation script:
var H25 = "TextH25"; // field name for cell H25;
var H22 = "TextH22"; // field name for cell H22;
event.value = ""; // clear field value;
// test for non-zero and non-null divisor;
if(this.getField(H22).value != 0) {
// perform division and set the field value;
event.value = this.getField(H25).value / this.getField(H22).value;
} // end of divisor not zero or null