parallels4 key won't work with parallels5 free upgrade

I purchased parallels 4 as indicated:

"Reference number: 293480595 | Order date: 06-SEP-2009 [Invoice/Order confirmation]
Pos. 1. Parallels Desktop 4.0 for Mac (#300134581)

Dear Peter ,

YOUR ACTIVATION KEY: HPH7.............................. etc"

However after upgrading to parallels 5 as soon as it was offered, which I understood to be free, after entering the license key I get the message that its invalid.. Can anyone help?? As you can see, parallels 4 was only purchased on 6th sept last year

Thanks

Peter