Glitches with the coupon functionality.

While testing the coupon code functionality, sometimes when I apply the coupon code then the subscription information + subscribe button disappears. It is the pop-out register box, so if I close it, re-open, and re-add the code, then it works fine. Also, on a separate test I received an error that the coupon is not valid for that subscription. I then close and re-open and it works fine every other time. Any idea of why this is happening and why only on occasion?