Automatic Core Data migration fails on Mac OS X 10.5 but not on 10.6 or 10.7

I've got a NSPersistentDocument-based Core Data app that targets 10.5 Leopard and above. I'm about to release an update that makes changes to the data model, and therefore need to migrate existing documents to the new model. The changes are relatively straightforward, and I've created a mapping model for them. Note that I'm not trying to do automatic lightweight migration, I do have a mapping model (lightweight migration isn't supported on Leopard, but my model changes rule it out anyway). In my NSPersistentDocument subclass, I override -configurePersistentStoreCoordinatorForURL... as follows:

- (BOOL)configurePersistentStoreCoordinatorForURL:(NSURL *)url
                                           ofType:(NSString *)fileType
                               modelConfiguration:(NSString *)configuration
                                     storeOptions:(NSDictionary *)storeOptions
                                            error:(NSError **)error
{

    NSMutableDictionary *newOptions = (storeOptions ?
                                       [NSMutableDictionary dictionaryWithDictionary:storeOptions] :
                                       [NSMutableDictionary dictionary]);
    [newOptions setObject:(id)kCFBooleanTrue forKey:NSMigratePersistentStoresAutomaticallyOption];
    return [super configurePersistentStoreCoordinatorForURL:url
                                                     ofType:fileType
                                         modelConfiguration:configuration
                                               storeOptions:newOptions
                                                      error:error];

}

This works fine on 10.6 and 10.7. However, on 10.5, the call to [super configurePersistentStore...] throws an exception and fails. The error is:

Error Domain=NSCocoaErrorDomain Code=134020 UserInfo=0x15812d70 "The model configuration used to open the store is incomp