Sims Medieval Game Code Problems

Hi all been going through all my old pc games and found a copy of sims medieval. I though brill, i'll give it ago not played it in a few years. Have tried to install the disc and when I get to inputting the game code 5 boxes come up on the screen to input the code, the problem is though on my disc copy there are only 4 boxes of game code on the leaflet that came with the game so obviously it keeps saying its an invalid code. Anyone got any ideas on what to do????