Applications :: Creating ITunes Store Account - Unable To Accept Valid Security Code?

I'm trying to make an iTunes Store account with my mother's MasterCard. Everything in the setup process goes well until I hit the part when I must enter in my three-digit security code. I do so, then proceed. iTunes won't proceed because it appears I have not entered a valid security code. But the problem is, it is in fact valid: I've used it before for other things and it works fine. I've triple-checked; it's kinda hard to incorrectly type a three-digit number.