Upgrade from CS3 to CS5 - won't accept my CS3 serial number

I own a copy of Fireworks CS3, and I purchased an upgrade copy of CS5 for my new Windows 7 computer. When I go to upgrade and enter the serial number (checked very carefully) from my CS3 package, the system says the number isn't valid. Not sure what to do. . . Any suggestions would be appreciated.

Thanks.

Dana