Promo code invalid

Hi,

I received a 25% off promo code by email on my birthday which should be valid until 31/3/2013. I'm trying to buy the Armoured Kill DLC but when i enter the code i get a message saying invalid code. I've checked the small print at the bottom of the email and i think it should be ok, but it isn't. Any idea what's wrong?

Cheers,

Dave