It Says The Code Is Invalid For The Sims 3 SIMS 3 SERIAL CODE = INVALID????!!!!!?

SIMS 3 SERIAL CODE = INVALID????!!!!!? - it says the code is invalid for the sims 3

I've had my Sims 3 game for a few months and I realized that they can register online, but when I try it says that the serial number is invalid. I wrote in "serial number" on the back of "The Sims 2 booklets in the DVD case.
This is the summary of my code:
L = LETTER
N = number

LLLN-LLLL-LLLL-NLLL-LNLL

Please help! Why is it valid?