iTunes unable to restore backup from iOS 8.2 beta 5 to iOS 8.3 beta 2

I'm updating from iOS 8.2 beta 5 to iOS 8.3 beta 2, by restoring from the .ipsw file for iOS 8.3 b2 for my device, and then using the backup created by iTunes when it was at iOS 8.2 b5. I get this error every time: "iTunes could not restore the iPhone "iPhone" because the backup was corrupt or not compatible with the iPhone that is being restored." There's nothing wrong with the backup. Whenever I restore back to iOS 8.2 beta 5, the backup works just fine. The backup is also not encrypted. I also tried going back to iOS 8.1.3 (the production version), and I get the same message. So right now, I'm stuck unable to upgrade or downgrade