iphone – 如何在UIPageViewController中设置第一页

*** TerminaTing app due to uncaught exception 'NSInvalidArgumentexception',reason: 'the number of view controllers provided (0) doesn't match the number required (1) for the requested transition'

这是我的代码.返回contentViewController而不是nil时,它工作正常.我该怎么办而不是零？

- (UIViewController *)pageViewController:(UIPageViewController *)pageViewController 
  viewControllerBeforeViewController:(UIViewController *)viewController {

    contentViewController = [[ContentViewController_iPhone alloc] init];

    int currenTindex = [self.modelArray indexOfObject:[(ContentViewController_iPhone *)viewController labelContents]];

    if ((currenTindex == 0) || (currenTindex == NsnotFound)) {

        //if on the first page,can't go BACk
        return nil;

     }

    contentViewController.labelContents = [self.modelArray objectATindex:currenTindex - 1];

    return contentViewController;

}