Minor Glitch in Calculate Shipping Button

This is a very minor issue but it may cause some confusion.

I was testing the Shipping Calculation on one of my auctions for shipment to the UK. Everything looked fine so I switched back to "Calculate Shipping to:" to the United States. My zip code was displayed in the zip box so I clicked the Calculate button and nothing happened.

I found that even though my zip is displayed by default, in order for the Calculate button to work, I have to manually enter it again.

I use Google Chrome and XP.