I have a coupon that I've added both to Google Checkout, and in my Zen Cart admin. However, when I do a test purchase with Google Checkout, it keeps telling me that my coupon code is invalid. I've tried uppercase and lowercase letters, so that's not the issue.

The "read me" file for the Google Checkout add-on says that coupons can be added, so I don't think that's the problem.

Any ideas?