iPhoneSetAnimationCallback, App doesn't return correctly from background

I’m currently adding GameCenter support to an iOS/SDL game and have, following the iOS Readme, moved my main function to a callback using iPhoneSetAnimationCallback.

When first starting the game this works fine, but if I put the game in the background then the engine doesn’t restart properly when brought to the foreground. I’ll get audio and frozen screen and then it crashes.

Do I need to do anything else? The readme only lists iPhoneSetAnimationCallback.
The only thing I’ve changed is to move the main code to a callback.