Forza 4 Unicorn cars error, Need help

I try to add the “Forza 4” Unicorn cars to my profile, But it comes up with this error message.

That say’s:
An error has occurred while writing back the load package!
If you see a developer, the following message will help him resolve the issue:

Index was out of range. Must be non-negative and less then the size of the collection.
Parameter name: Index