Points Greater Than Total

The module is working flawlessly, except for one little thing. If a user has 100 points (where 1 point = $1), and they buy something worth less than 100, but still put in 100 for their userpoints discount, it results in a negative total.

i.e., here is the total from one transaction:

Subtotal: $32.50
Discount order using Points: -$90.00
Standard Flat Rate: $6.00
Subtotal excluding taxes: -$51.50
Texas Taxes: $3.18
Total: -$48.32

Any ideas?