NSEntityDescription对象未在ios 6.1.3中创建

Question

NSEntityDescription在Simulator iOS 6.0中工作正常，但我无法在ios6.1及更高版本的iPhone设备上运行我的代码由于未被捕获的异常“NSInvalidArgumentException”，它会给出例如终止应用的异常，原因：executeFetchRequest:error: A fetch request must have an entity.

我的代码是：

NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];

NSEntityDescription *entity = [NSEntityDescription entityForName:@"Wager" inManagedObjectContext:self.managedObjectContext];

[fetchRequest setEntity:entity];