Discount amount off 'exceeds' price being paid - but in fact equals cart amount

We have a store with digital and physical products. The digital albums are typically $15. We have a few customers we have given a $30 code to pick some digital products. Our customers have picked 2 digital and now on checkout they get an error message:

Discount Code cannot exceed the price being paid.

The price being paid is exactly equal to the discount code so this shouldn't be happening.

Already a tad annoyed we have no access to 'advanced discounts' with a site that pays for member areas, commerce and emails.

How can we help this customer get through checkout smoothly! this clunky ness is very annoying. 

kindly

Vibeke