EXCEL Formula that calculates price after percentage discount, but needs to ignore a "-" to show no discount =IF(I40"-",(I39-(I39*I40)))

So I have a formula for adding subtotal after a percentage discount is taken off.(I39-(I39*I40)

I also have a dropdown list for percentages so 10% 15% 20% and none labeled as -

This is what I have =IF(I40<>"-",(I39-(I39*I40)))

The problem with this is that the formula comes back as False when - is chosenI need it to calculate the percentage off, when there is one (10%), and show the total when there isn't one (-).