I'm guessing that the view controller with the managedObjectContext property is embedded in a navigation controller. In the segue method, make sure you grab a reference to the correct view controller:
- (void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender
{
if ([[segue identifier] isEqualToString:@"MySegue"]) {
UINavigationController *navController = (UINavigationController *)segue.destinationViewController;
MyViewController *vc = (MyViewController *)navController.topViewController;
AppDelegate *appDelegate = [UIApplication sharedApplication].delegate;
vc.managedObjectContext = appDelegate.managedObjectContext;
}
}