Userpoints Payment and UC Coupon isn't working

Issue Summary

A user has a cart with one product worth $14. It is shippable for $1.99, bringing the normal total to $15.99. This user enters their billing and shipping information and the shipping is calculated appropriately. Then they enter a coupon code for $5 off, dropping the total to $10.99. They choose to pay with $12 in Userpoints (all they have available), but then they get an error saying that they do not have enough Userpoints to complete the transaction. The user is returned to the checkout page. Repeating the process results in the same error.

I've seen this before, but I haven't found any suggestions in the Issue Queues that were helpful (perhaps I missed them). Does anyone else experience this issue or know what might be going on?

If I find more details I'll add them to this issue.

Thanks!