Coupon for $5.99 On Demand Movie or Credit

I received a coupon in the mail to "Take $5.99 off your next Xfinity On Demand movie rental or purchase.  Or simply use it as a credit towards your Xfinity bill."  To redeem, it says to visit my.xfinity.com/coupon and follow the easy on-screen steps to activate the coupon.  When I got to that site, I am asked for a 16 digit code to activate the coupon.  The problem is there is no coupon code and my account is showing I have no credits. How do I go about this??