Conditional requirement field not working

Thanks in advance. I can't figure out why this isn't working. If number field "Part 3-1-Quantity" is greater than zero, then date field "Part 3-1-Expiry Date" should be required.

var quantity = this.getField("Part 3-1-Quantity").value;
var expiryDateField = this.getField("Part 3-1-Expiry Date");

if (quantity > 0) {
expiryDateField.required = true;
} else {
expiryDateField.required = false;
}